I don't know if there's any official boxing rule saying that a boxer needs to drop to the ground (or kneel etc) before he can be considered knocked down and the counting can begin, but if it's up for the referee to decide, then I see no controversy re the round 9 situation.
Tyson was in trouble but he was conscious, so stopping the fight would be completely wrong, and I don't think Fury would necessarily be KO'd cold if the referee didn't step in. Anything could've happened, but, to me, it looked like Fury was about to drop on his knee before he noticed the ref got between himself and Usyk.
This doesn't matter though, as Usyk has still won, and during the post-fight interview, he wasn't bothered by the situation at all.
I haven't seen the full fight yet, only the extensive highlights, but think Usyk won fair and square. The rematch should be really exciting.