To be fair, he didn't assert this to be true; he merely proposed that as an option to calling Jesus a liar. Which is no different than the "lord/liar/lunatic" argument famously outlined by C.S. Lewis. Which is itself a false di- (or tri-) -chotomy, but that is another discussion.
This is one of the possibilities:
either he didn't exist, or he was insane (sufferer of an early case of Jerusalem syndrome) or he was just a rebel leader and his followers added the whole God mishmash...
What is sure of course is that he wasn't the messiah or even a messiah...