"Commitment mechanism" typically means some way to impose a cost on a party for breaking the commitment, otherwise it is, in the game theorist's parlance, "cheap talk" instead. In the one-shot PD, there is by definition no commitment mechanism, and it was in this LCPW that Eliezer's decision theories are frequently tested.

You're talking about the repeated PD with "always cooperate," rather than the one-shot version, which was the scenario in which we found ourselves with Clippy. Please understand--I'm not saying EU-maxing agents do not have commitment mechanisms in general, just that the PD was formulated expressly to show the breakdown of cooperation under certain circumstances.

Regardless, always cooperate definitely does not maximize expected utility in the vast majority of environments. Indeed, it is not part of literally any stable equilibrium in a finite-time RPD. But more to the point, AC is only "committed" in the sense that, if given no opportunities afterward to make decisions, it will appear to produce committed behavior. It is unstable precisely because it requires no further decision points, where the RPD (in which it is played) has them every round.