Is it? What if the AI is better than a grandmaster chess player and is generating the most likely next move that a grandmaster chess player might make and not the most likely move to win, which may be different?

Depends on the training data I think. If the data divides in games by top chess engines - and human players, then yes, it might make a difference to tell it, to play like a grandmaster of chess vs. to play like the top chess engine.