Not surprising at all. He did a good job of keeping his feet up initially, but with all of that bouncing, he was bound to catch one of them. The hard plastic boots used will usually protect the ankle, but gives you a mid shin break.
I don't know if I would have been able to self arrest at that speed either. Whenever I am out, I always find a safe area to slide and practice self arresting while sliding on stomach/back both feet first and head first. Don't know if I would have the composure at that speed to do it.
One major mistake I saw there is that I looks like he stays on his back. First thing you are supposed to do is get on.your stomach. It makes it easier to keep your feet up and to he able to get to your axe.