You are asking to prove a negative. But even assuming that the model is capable of returning every bit of its training data verbatim (a mathematical impossibility) that would not be enough as mere capability is insufficient here. If capability alone were the standard any library that also has a photocopier / scanner would be in violation.
To prove distribution of copyrighted materials it would have to be practical and actually used in the wild by people to circumvent copyright and generate copies of those works. Again, I can't prove a negative, but that isn't the standard, and nobody has shown a practical exploit here.
There was a paper a while back where (from memory) they managed to coax 75% of the original text of some internet-popular books out of an LLM. Harry Potter, 1984, etc. That's why I said it was possible for some texts.
My assumption is that multiple copies in the training data "wear a deeper groove". I believe those are infringing, and should be dealt with on a case-by-case basis. But the vast majority of text doesn't wear that groove.
(Edit: Think it was this one https://arxiv.org/abs/2601.02671)