For the past few years, NCAA officials have allowed the principle of verticality area for defenders - and they should. Our bigs use the defensive maneuver quite effectively, btw.
That said, the only contact between Pons and Clarke on that play was body contact. Clarke never touches him in any other way, imo (I've watched it in slow motion multiple times - I really wanted it to be a foul.)
So the question becomes, should Clarke have been called for the body contact. I don't think so. Pons' momentum toward the rim is what caused the contact while Clarke, while not completely vertical, was in good defensive position imo.
As much as it pains me to say it, that was just an incredible rejection.
I thought it was clean, too. Perhaps not worthy of the oohing and awing that went on and on from our illustrious TV commentators, but a clean block nonetheless.