How can this be? If nobody plays a weakly dominated strategy, then everyone with type above the reserve price is bidding their value, and so I'm not successfully ironing—meaning the induced allocation rule is suboptimal in the irregular case.

But the key observation is that *some* equilibrium makes this mechanism optimal. Indeed, recall that an optimal allocation rule gives the good to whoever has the highest ironed virtual value (IVV) if that value is positive. But there could be many such rules!