I, too, was distracted by the mention of the prisoners' dilemma, especially before the social situation was made clear, but that doesn't mean it wasn't a good rhetorical technique.

One answer to your question is that it's a repeated PD. Being nasty may produce an advantage in the short term, but it takes attention from positive-sum cooperation, the ostensible purpose of the conversation.

Another point is that it is the game has incomplete information. Alice may be surprised by Bob's apparent belief that he can gain advantage, but it's a pretty strong signal.

But I think quite often the situation is that it is quite clear to Alice that Bob is confused and she records him not as a defector, but as a loose cannon who can't even backstab properly, let alone do complicated things like cooperate.