Trees, on the other hand, are scary.
> most of the circulation he's stuck in looks to be horizontal
This section has up to 60m depth, the underwater currents are extremely turbulent and will you pull you under the rock on the sides (the whitewater is just a 'slit' on the surface, there's a whole river running underneath). If he didn't have someone ready to pull him out in two seconds like in the video, it's a good bet he wouldn't make it out.
It's possible that the man depicted could have been dragged under an undercut if he'd swum the full rapids, but the video doesn't show evidence of that. Rather, he's pulled out of an eddy without much drama.
(Whilst anyone unprepared, or a daredevil, wouldn't have had a life jacket, and then maybe 100%)
They're also less predictable than a lot of other river hazards. It's possible to come around the bend and be faced with a big strainer that wasn't there the last time you ran the river.
Here's a good video showing a rescue from one: https://www.reddit.com/r/whitewater/comments/cn6k8k/nasty_st...
Thanks for the explanation and link