Why do you think they can't say how they did on the previous season? As others have mentioned, James has talked about being "The Dumbest Survivor ever" and everyone else obviously knew and understood the reference, meaning he told them the whole story, which meant he had to have told a LOT about that season.

As for someone saying Amanda was "cheating" by telling Parvati--if anything, it would be worse for her chances once P knew that.

Mostly, I believe it is very naive of you to think they spent 38 days 24/7 together, with references to people's previous seasons made constantly, and think that nobody would find out about the China outcome.