If you've checked the author's previous post [1], you'll see that he admits to being 1200 Elo on chess.com, which is beginner / early-intermediate level. So him losing to GPT-4 may not mean much. Maybe that explains the supposed contradiction here.
The problem with ChatGPT seems to be that it often gives answers that appear plausible at the surface, but with enough knowledge you realize they are inaccurate or even wrong. I wouldn’t put much stock in the analysis of a beginner - I’d trust them to say the moves were legal, and that ChatGPT stopped trying to materialize pieces from thin air, but not any analysis beyond that.