Player 2 observes "not A" as a choice. Doesn't player 2 still need to estimate the relative probabilities that B was chosen vs. that C was chosen?

Of course Player 2 doesn't have access to Player 1's source code, but that's not an excuse to set those probabilities in a completely arbitrary manner. Player 2 has to decide the probability of B in a rational way, given the available (albeit scarce) evidence, which is the payoff matrix and the fact that A was not chosen.

It seems reasonable to imagine a space of strategies which would lead player 1 to not choose A, and assign probabilities to which strategy player 1 is using. Player 1 is probably making a shot for 6 points, meaning they are trying to tempt player 2 into choosing Y. Player 2 has to decide the probability that (Player 1 is using a strategy which results in [probability of B > 0]), in order to make that choice.