Those guys are just plain idiots.
Kayakers who truly understand paddling Big Water wouldn’t paddle that, because of what you said - all the flotsam and debris. It doesn’t matter how good a paddler you are if you get run over from behind by a 4,000 pound tree that popped up behind you as you were picking your line around a standing wave.
Also, experienced paddlers don’t attempt Big Water without rescue in place from other paddlers. They back each other up on runs through big rapids and wave chains. One will go first, then hang in an eddy with a throw bag, and they’ll leapfrog each other all the way down the river. Those idiots were just solo paddling together. No rescue plan at all.
Darwin was paddling right alongside those guys. They got really lucky.