That's one of the rarest occurrences in MMA. You'll see the odd upkick KO from a fighter on his back (Mousasi's on Jacare was brilliant, something he trains), how often do you see some dude mounted KO the dude on top of him with a punch? Pretty rare situation. Perfectly placed punch, too, and a stoppage the other way was an eventuality.
I'd say a mounted KO from off his back for the boxing interim champ is an all-timer. It won't even probably win KO of the year, though, just bc the shit is a popularity contest.