Ok I watched the slowmo. Here's what I see, as a clueless gaijin, might be different to what others see.

Ao had Ono's belt with his left hand. Ono steps forward with right leg, maybe bumps Ao's leg maybe not, but Ono slips with that right foot and falls towards. Ono's falling momentum skittles Ao's legs out from under him, so they're both falling towards each other with forward rotation, but Ono has more forward momentum, having started falling earlier and being closer to horizontal already, so Ao is flying backwards while rotating forwards, and as we saw his feet eventually land outside. Ao didn't jump, he was knocked back, feet first. So my idea of his belt grip enabling him to pivot forwards was far-fetched. He was already out of control.

So Onosato fell first. Hit the ground first. And whether thru skill or yokozuna luck he managed to execute a winning move while falling, launching Aonishiki in such a way that he couldn't land in the ring.

I still think it should have been a rematch, because Ono was already falling out of control with no way of landing on his feet when he skittled Ao.

Maybe the real question is why do I care so much about this bout? Must be something about sumo...