Personally I think the illegal moves are irreverent, the fact that it doesn't play exactly like a typical 1400 doesn't mean it can't have a 1400 rating. Rating is purely determined by wins and losses against opponents, it doesn't matter if you lose a game by checkmate, resignation, or playing an illegal move.
That's not to say ChatGPT can play at 1400, just that that playing in an odd way doesn't determine its rating.
