It answers that question by replacing the Prisoner's Dilemma with an entirely different game, which doesn't have the awkward feature that makes the Prisoner's Dilemma interesting.

Wei_Dai (if I've understood him right) is not claiming that an SI's claim on its source code constitutes a PD, but that one obvious (but inconvenient) way for it to arrange for mutual cooperation in a PD is to demonstrate that its behaviour satisfies the condition "I'll cooperate iff you do", which requires some sort of way for it to specify what it does and prove it.