NCAA Men's Championship match

[Scrapper1]

Did anybody see the Championship match last weekend?

Take a look at this play.

https://youtu.be/2JJFf8evDWs?t=5357

I was watching it live and didn't understand what was happening. This is a clear back row block that is missed. In fact, you can see the R1 signaling that the back row player was below the height of the net (which you'll see that he wasn't). But then the play is challenged. What did the Hawaii coach challenge??? You can't challenge whether the ball was above the height of the net. But the challenge is accepted and then is successful.

I had to watch it about 10 times to understand what happened. Thoughts/comments?