He accepted it. He's still thinking from inside the confines of those two religions. They said that God was unknowable, and he accepted that as part of his argument.
In truth, we only know that ancients were wrong about something or everything. They could just as well be wrong about what they thought God was, being unknowable, as they were about the existence of a God.
Its really odd to say "no, you were completely right about this part but it just so happens that everything that the parts are connected to does not exist"

