This argument is pretty flimsy. ChatGPT makes illegal moves frequently. In all my years of playing competitive chess (from 1000 to 2200), I have never seen an illegal move. I'm sure it has happened to someone, but it's extremely rare. ChatGPT does it all the time. No one is arguing that humans never make illegal moves; they're arguing that ChatGPT makes illegal moves at a significantly higher rate than a 1400 player does (therefore ChatGPT does not have a 1400 rating).
Edit:
Without reading everything again, I'll assume someone said "never." They're probably assuming the reader understands that "never" really means "with an infinitesimal probability," since we're talking about humans. If you're trying to argue that "some 1400 player has made an illegal move at some point," then I agree with that statement, and I also think it's irrelevant since the frequency of illegal moves made by ChatGPT compared to the frequency of illegal moves made by a 1400 rated player is many orders of magnitudes higher.
Edit: Without reading everything again, I'll assume someone said "never." They're probably assuming the reader understands that "never" really means "with an infinitesimal probability," since we're talking about humans. If you're trying to argue that "some 1400 player has made an illegal move at some point," then I agree with that statement, and I also think it's irrelevant since the frequency of illegal moves made by ChatGPT compared to the frequency of illegal moves made by a 1400 rated player is many orders of magnitudes higher.