But we could tell a story, of Omega meeting a TDT agent who one-boxes and gets the reward, in which everything is caused by forward-in-time physical causality.

Many things can be explained in multiple different ways, and for physical events a physical (causal) explanation is always possible. The lesson of LW-style decision theories seems to be that one shouldn't privilege the physical explanation over other types of knowledge about how events depend on each other, that other kinds of dependence can be equally useful (even though there must be a physical explanation for how those dependencies between physical events got established).