I would say it actually is possible. Chances for something like this to happen are small, but it would be possible. I guess noone of us has any info about this, so it's all pure speculations, but let's say he hit to rock at almost 90 degree angle and he his it straight with camera. Camera probably did brake away, but most likely camera mount on helmet, and some part of mount where screw goes through connecting mount and Gopro housing was left there. So if he hit rock exactly with that spot, forces concentrated to that little piece were much bigger as they would be if he would hit with bigger surface of helmet. In worse case, this could even make mount penetrate helmet (if I remember right, there were some reports helmet did fall apart in crash).
So yes, it's possible even though chances are pretty small I would say, and no, "there's lot of athletes using them and nothing ever happened" is not relevant. Only thing with this statement is, that noone famous enough to get world wide coverage got hurt this way. Noone really knows how many normal people who will never get into newspapers got hurt this way. Maybe none, or maybe, there's 100s or 1000s of them around the world. Just noone reports if Average Joe gets hurt this way, so we might think there was noone ever got hurt this way.