I've heard this is an assumption. I've never heard it backed by anything. The argument was made to me 25 years ago in reference to anime/drawings of CSAM, suggesting that paedophiles would "settle" for the fake and not harm children.
I assume — I have no proof either way — it would just normalise their behaviour and they would continue to offend. I feel like you could give them access to everything that's already been made (even the real stuff, for the sake of the argument) and they would still want to harm actual children. I don't think fuelling their fantasies is going to make them stop trying to fulfill them.