That bull was injured by another bull before the grizz found him, he probably wouldn’t of made it too long with all the predators around. I saw a bull bison that had broken his lower rear leg in a fight with another bison last years august And within 2 days the wolves finished him off. Injured animals don’t last long out there.
Would be interested whether you know for sure if that's what happened?
I suggested on Facebook, after seeing the video there, that the elk could either have been injured in a fight with another bull (have seen that many times in Yellowstone this time of year) but since bulls tend to be pretty rut-dumb in late September, the bear may also have stalked and grabbed him, then gotten away just before the video.