To be clear, in my VERY untrained medical opinion, the mechanism of injury here is completely different from what is often referred to as "harness hang syndrome".
It's not entirely clear what happened, but it sounds like he was head down in the waterfall.
So as Junkman points out, it's very likely hypothermia was the real killer. If that's not the case, it is more likely that being head down for so long increased intracranial pressure.
What we normally think of as "harness hang syndrome" is more likely due to reduced blood flow to the brain. I refer to
Dr. Mortimer's piece for those who have not read it. (And if you're lucky and can hear him talk about it, all the better.)