> In other words, if full determinism removes agency from the perpetrator, then it also removes it from everyone else.
This is irrelevant to their claim. Normative statements don't require anyone actually have agency. After all, it's possible that everyone should do something that nobody can do.
If you want to say that fine then their claim is irrelevant too, then sure, but it's in no way inconsistent. I think it is relevant though. We can do things differently than we do them, like after reading this comment section; we just can't do them differently than we will do.
Worse still. Your guilty of the same thing you're accusing the poster of.