The assumption of rationality is usually used to get a tractable game. That said, the assumption is not as restrictive as you seem to say. A rational sorcerer isn't obliged to cooperate with you, and can have other goals as well. For example, in my game we could give Dark a strong desire to move the ace of spades to the top of the deck, and that desire could have a certain weight compared to the desire to stay hidden. In the resulting game, Daisy would still use only the information from the deck, and wouldn't need to do Bayesian updates based on her own state of mind. Does that answer your question?