tldr: in sumo, if person A is flying out of the ring, and person B who is inside the ring falls down first, person B still wins.

So.... I'm not an expert, but I think this is the explanation:

Shini-tai (死に体) is a term used in sumo wrestling. In general, the first sumo wrestler to touch any body part outside the ring, or have any part of his body other than the soles of his feet touch the ground loses. There are exceptions to the rule, shini-tai being one of them.

Literally meaning 'dead body' or 'dying body,' the term is used to describe a wrestler who was not first to fall or touch outside the ring, but who had no chance of winning owing to the superior technique of his opponent (and his falling position).[1] Whilst a relatively rare event, it is most often seen after close bouts in which one wrestler clearly had the advantage towards the end.

https://en.wikipedia.org/wiki/Shini-tai

So, consider the very last moment just before Onosato hit the floor.

At that moment, Aonishiki was in the air, outside of the ring, and there was no way Aonishiki could have landed on his feet inside the ring. The gyoji in the ring observed this and declared Onosato the winner, and one of the ringside referees could have objected if they disagreed with the call.

HOWEVER ... this is not very satisfying, because it seems like Aonishiki was in charge the whole time, controlling Onosato with his left hand and pushing him down with his right. As @not_woody_shaw@lemmy.world points out, Onosato was also thrust down because of Aonishiki's technique.

BUT... I tentatively think that the referees made the right call. Aonishiki was flying backward for a reason, namely Onosato's forward motion. So the fact that Aonishiki did a technique on Onosato as Aonishiki was being thrust out of the dohyo isn't really relevant; there was no way for Aonishiki to win, because he could not and did not land inside the ring. That's just one of the rules of sumo.

CONSIDER: if Aonishiki had done an Ura-like roll right over Onosato and landed on his feet inside the ring. That would have been epic. Or even landed on top of Onosato inside the ring and then climbed off. Unfortunately, Aonishiki had no forward momentum.

Conclusion: it was a close match, and it would have been more satisfying to the audience if the referees had conferred, but I think they made the right call.