Regarding your confusion on what Lewis may have meant by turning things into nature, I may have some insight.
Lewis, being a classicist, was well versed in Aristotle's four causes. One critique of reductionism is that it focuses on one of the four causes, the material cause, and leaves out the other three. And I think it is reductionism that Lewis is referring to: the philosophy that breaks things down into parts and then says "That's what the thing is." The whole topic makes me think of the exchange between Aslan and Eustance in The Voyage of the Dawn Treader:
"In our world," said Eustace, "a star is a huge ball of flaming gas."
"Even in your world, my son, that is not what a star is, but only what it is made of."
It also brings to mind another passage, this one from his earliest book authored as a Christian, "The Pilgrim's Regress". At one point the main character, John, has been rescued from a giant called Zeitgeist by a woman warrior named Reason (the book is not a subtle allegory to be sure). While he was held prisoner by the giant, every time the giant opened his eyes and looked at John or the other prisoners their skin would turn clear and you could see all the guts and organs inside. After the rescue Reason and John have the following exchange:
"'Did you think that the things you saw in the dungeon were real: that we really are like that?’
‘Of course I did. It is only our skin that hides them.’
‘Then I must ask you the same question that I asked the giant. ‘What is the colour of things in the dark?’
‘I suppose, no colour at all.’
‘And what of their shape? Have you any notion of it save as what could be seen or touched, or what you could collect from many seeings and touchings?’
‘I don’t know that I have.’
‘Then do you not see how the giant has deceived you?’
‘Not quite clearly.’
‘He showed you by a trick what our inwards would look like if they were visible. That is, he showed you something that is not, but something that would be if the world were made all other than it is. But in the real world our inwards are invisible. They are not coloured shapes at all, they are feelings. The warmth in your limbs at this moment, the sweetness of your breath as you draw it in, the comfort in your belly because we breakfasted well, and your hunger for the next meal—these are the reality: all the sponges and tubes that you saw in the dungeon are the lie.’
‘But if I cut a man open I should see them in him.’
‘A man cut open is, so far, not a man: and if you did not sew him up speedily you would be seeing not organs, but death. I am not denying that death is ugly: but the giant made you believe that life is ugly.’
‘I cannot forget the man with the cancer.’
‘What you saw was unreality. The ugly lump was the giant’s trick: the reality was pain, which has no colour or shape.’
‘Is that much better?’
‘That depends on the man.’
‘I think I begin to see.’
‘Is it surprising that things should look strange if you see them as they are not? If you take an organ out of a man’s body—or a longing out of the dark part of a man’s mind—and give to the one the shape and colour, and to the other the self-consciousness, which they never have in reality, would you expect them to be other than monstrous?’
‘Is there, then, no truth at all in what I saw under the giant’s eyes?’
‘Such pictures are useful to physicians.’"
All this to say that Lewis was opposed to reductionism, and science is often reductive. I do not believe that he meant that scientific understanding changed actual reality, but it can certainly change our understanding of reality. But I doubt Lewis believed this reductive "scientific" understanding of reality was true. See also this blog post by Edward Feser (https://edwardfeser.blogspot.com/2023/11/ryle-on-microphysics-and-everyday-world.html).
I believe we see this in The Abolition of Man itself, such as in this passage:
"Nature is a word of varying meanings, which can best be understood if we consider its various opposites. The Natural is the opposite of the Artificial, the Civil, the Human, the Spiritual, and the Supernatural. The Artificial does not now concern us. If we take the rest of the list of opposites, however, I think we can get a rough idea of what men have meant by Nature and what it is they oppose to her. Nature seems to be the spatial and temporal, as distinct from what is less fully so or not so at all. She seems to be the world of quantity, as against the world of quality; of objects as
against consciousness; of the bound, as against the wholly or partially autonomous; of that which knows no values as against that which both has and perceives value; of efficient causes (or, in some modern systems, of no causality at all) as against final causes. Now I take it that when we understand a thing analytically and then dominate and use it for our own convenience, we reduce it to the level of `Nature' in the sense that we suspend our judgements of value about it, ignore its final cause (if any), and treat it in terms of quantity"
Note that he's speaking here of how we understand and treat "Nature" not reality itself. He seems focused on how we view reality, not that our understanding of reality changes it.
Regarding your confusion on what Lewis may have meant by turning things into nature, I may have some insight.
Lewis, being a classicist, was well versed in Aristotle's four causes. One critique of reductionism is that it focuses on one of the four causes, the material cause, and leaves out the other three. And I think it is reductionism that Lewis is referring to: the philosophy that breaks things down into parts and then says "That's what the thing is." The whole topic makes me think of the exchange between Aslan and Eustance in The Voyage of the Dawn Treader:
"In our world," said Eustace, "a star is a huge ball of flaming gas."
"Even in your world, my son, that is not what a star is, but only what it is made of."
It also brings to mind another passage, this one from his earliest book authored as a Christian, "The Pilgrim's Regress". At one point the main character, John, has been rescued from a giant called Zeitgeist by a woman warrior named Reason (the book is not a subtle allegory to be sure). While he was held prisoner by the giant, every time the giant opened his eyes and looked at John or the other prisoners their skin would turn clear and you could see all the guts and organs inside. After the rescue Reason and John have the following exchange:
"'Did you think that the things you saw in the dungeon were real: that we really are like that?’
‘Of course I did. It is only our skin that hides them.’
‘Then I must ask you the same question that I asked the giant. ‘What is the colour of things in the dark?’
‘I suppose, no colour at all.’
‘And what of their shape? Have you any notion of it save as what could be seen or touched, or what you could collect from many seeings and touchings?’
‘I don’t know that I have.’
‘Then do you not see how the giant has deceived you?’
‘Not quite clearly.’
‘He showed you by a trick what our inwards would look like if they were visible. That is, he showed you something that is not, but something that would be if the world were made all other than it is. But in the real world our inwards are invisible. They are not coloured shapes at all, they are feelings. The warmth in your limbs at this moment, the sweetness of your breath as you draw it in, the comfort in your belly because we breakfasted well, and your hunger for the next meal—these are the reality: all the sponges and tubes that you saw in the dungeon are the lie.’
‘But if I cut a man open I should see them in him.’
‘A man cut open is, so far, not a man: and if you did not sew him up speedily you would be seeing not organs, but death. I am not denying that death is ugly: but the giant made you believe that life is ugly.’
‘I cannot forget the man with the cancer.’
‘What you saw was unreality. The ugly lump was the giant’s trick: the reality was pain, which has no colour or shape.’
‘Is that much better?’
‘That depends on the man.’
‘I think I begin to see.’
‘Is it surprising that things should look strange if you see them as they are not? If you take an organ out of a man’s body—or a longing out of the dark part of a man’s mind—and give to the one the shape and colour, and to the other the self-consciousness, which they never have in reality, would you expect them to be other than monstrous?’
‘Is there, then, no truth at all in what I saw under the giant’s eyes?’
‘Such pictures are useful to physicians.’"
All this to say that Lewis was opposed to reductionism, and science is often reductive. I do not believe that he meant that scientific understanding changed actual reality, but it can certainly change our understanding of reality. But I doubt Lewis believed this reductive "scientific" understanding of reality was true. See also this blog post by Edward Feser (https://edwardfeser.blogspot.com/2023/11/ryle-on-microphysics-and-everyday-world.html).
I believe we see this in The Abolition of Man itself, such as in this passage:
"Nature is a word of varying meanings, which can best be understood if we consider its various opposites. The Natural is the opposite of the Artificial, the Civil, the Human, the Spiritual, and the Supernatural. The Artificial does not now concern us. If we take the rest of the list of opposites, however, I think we can get a rough idea of what men have meant by Nature and what it is they oppose to her. Nature seems to be the spatial and temporal, as distinct from what is less fully so or not so at all. She seems to be the world of quantity, as against the world of quality; of objects as
against consciousness; of the bound, as against the wholly or partially autonomous; of that which knows no values as against that which both has and perceives value; of efficient causes (or, in some modern systems, of no causality at all) as against final causes. Now I take it that when we understand a thing analytically and then dominate and use it for our own convenience, we reduce it to the level of `Nature' in the sense that we suspend our judgements of value about it, ignore its final cause (if any), and treat it in terms of quantity"
Note that he's speaking here of how we understand and treat "Nature" not reality itself. He seems focused on how we view reality, not that our understanding of reality changes it.