In my half century of watching wrestling, I've seen some wrestlers knees get bent more than 90 degrees sideways. In comparison, I did not see anything even close to that in this bout, and nothing worth calling until DeSanto screamed (not quite a scream, really).
I'm not doubting for a second DeSanto was hurt, I believe he was. I also don't believe, for a second, that RBY would EVER do anything to intentionally hurt his opponent.
There are two ideas I have, one is speculation, one is related to the rules. First, the way this played out, is it possible there was something pre-existing? Could DeSanto's knee have been tweaked in the room, or prior bout, and this was aggravating a pre-existing minor injury?
Second, I feel for the referees. The whole range-of-motion language in the rulebook doesn't help when each and every wrestler is different. We've all seen body parts moved in directions that suggest double-jointedness, while others have joints that don't have close to that flexibility. I know there are guidelines, but I believe this wrestler-to-wrestler difference can be a bit problematic.