You seem to be sure it's not a hack, but actually nothing can prove it. What you don't see on the video, is the second when he was going to die, I was trying to hit him but instead of taking the hit, some freeze occurred on his character and he was already far away as a teleport would do. Coincidence is very difficult to admit, because I've already met few guys in the past doing exactly the same thing in the same situation. And we were all knowing it was a cheat.
So, I can understand that it could be a bug but saying that it is absolutely, no. You don't have logs that can prove it, so you know nothing for the moment about this guy and what he has done exactly when I recorded the video.