I thought that part of it at least was that much like existing life has a harder time breaking down these mirror proteins the same is true in reverse. So any life that was mirror protein based would struggle with consuming and gaining energy. As such the current protein chirality basically won by being first to market. That being said I don’t see why that would hamper reverse chirality photosynthesis, and I don’t really know what I’m talking about so perhaps your suggestion is more convincing after all.
I mean, assuming the existence of a god, I don’t disagree. But if you are assuming the existence of a god then you’re not really atheist. This is the kind of misinterpretation that theists who can’t imagine not believeing in the supernatural often make about atheists; that we’re simply angry or confused rather than rejecting the paradigm altogether.