Earlier quoted context omitted.
I think the authors rule is fair. If we interpret illegal moves as getting stuck in an online game, the resulting Elo rating is what it would get. But ye, he is anthropomorphizing alot ...
There's no indication that GPT-3.5 was stuck when it tried to make illegal moves. GPT-4 clearly was making illegal moves when it was very much not stuck. It just doesn't know how to play, but the author decided to interpret it as frustration.
I meant stuck as "it can't make the move" and would timeout the game if playing online.