A well-designed optimization agent probably isn't going to have some verbal argument processor separate from its general evidence processor. There's no rule that says she either has to accept or refute humans' arguments explicitly; as Professor Quirrell put it, "The import of an act lies not in what that act resembles on the surface, but in the states of mind which make that act more or less probable." If she knows the causal structure behind a human's argument, and she knows that it doesn't bottom out in the actual kind of epistemology that would be neccessary to entangle it with the information that it claims to provide, then she can just ignore it, and she'd be correct to do so. If she wants to kill all humans, then the bug is her utility function, not the part that fails to be fooled into changing her utility function by humans' clever arguments. That's a feature.