To be fair, we have the benefit of that video so it's not just a case of "idiot must have lost it on a corner". It's pretty clear why he came off, just not apparently how it was triggered. Whatever the cause, his line in was a massive contributing factor as was gear choice, lean angle, loss of braking and control. 9 times out of 10 he might have got away with it, this time he didn't. I'm sure we've all cocked up in the past and thought ourselves lucky to have got away with it too, and videos like this just reaffirm the importance of defensive riding (even for faster riders), choosing the right lines, appropriate speeds and trying not to put yourself in a position where risk of accident and injury is increased.
Agreed, those injuries were surprisingly severe given the relatively low "off" speed but anyone flung into the air landing on rocks is going to hurt themselves badly. I did much the same on a MTB in Spain 10 years ago, breaking ribs and other bits of myself. Hope the guy makes a full recovery including his mental scars healing.