Elon claims a lot of crazy stuff, but do you have the original source in context?
Because autopilot is not autonomous and no one claims it to be. It e.g. cannot solve crossroads or react to traffic lights.
So I guess he either said that about FSD (which would still be questionable but at least not nonsense) or he meant that drivers using autopilot assistance have in general less crashes than drivers who don't use it.