Yeah, it's that "Dead Body Rule": https://en.wikipedia.org/wiki/Shini-tai which is about as popular as the "offsides" rule in football.

One one hand, if you look at the following image (which takes place just before the ones above) they're both in midair, and as you show above they both touch down outside the ring, Hakunofuji touching down first.

On the other hand, this situation is more due to Hakunofuji's push-out than Aonishiki's push-down, so it's due to Hakunofuji's "superior technique" in dead-body-rule terms.

So I think the call was right. But it looks like there wasn't even a review, even though gyoji seemed momentarily unsure. A review followed by a re-match would have been reasonable too.