It'll be revealed at EoS I bet, when Hashi is having some kind of flashback looking over Naruto. Ultimately though, if Kishi stops THIS fight to answer a relatively unimportant question like this, I'm going to be very upset.
I've wondered if his ability to create clones in another dimension is an EMS technique it does remind me of kamui. And there is that panel on the page where Madara was stabbed by Hashi that indicates something on the hillside. Maybe a clone.
However, the answer to how he survived doesn't seem relevant to the story line at the moment.