I interpreted it as saying, for him, as soon as he hears one of those words, the conversation is over.

Which… sounds like… if not heresy, then taboo?

He's saying that, when person A says something, and person B says that the statement is x-ist, person B intends that to end the conversation. In particular, person B intends that they not have to actually refute person A's statement.

PG's reaction isn't the point at all. The point is the speaker's intent.