And it has already been stated elsewhere in the thread: an illegal move is not technically a forfeiture, so this is some heavy "giving the benefit of the doubt".
It would be interesting to see how ChatGPT would play after making the first illegal move. Would it go off the rails completely, playing an impossible game? Would it be able to play well if its move was corrected (I'm not sure how illegal moves are treated in chess; are they allowed to be taken back if play hasn't progressed?). Could it figure out it made an illegal move, if it was told it did, without specifying which one, or why it was illegal? By stopping the game as soon as an illegal move is made, the author is missing the chance to understand an important aspect of ChatGPT's ability to play chess.
I got the impression the author did this because they thought they were being fair with ChatGPT, but they're much more likely to be letting it off the hook than they seem to realise.
(Sorry about the "they"'s; I think the author is a guy but wasn't sure).