Technique, flexibility, and comfort in the water are more important than upper body strength for distance swimming. That guy had none of those things. Add in low body fat (fat floats, muscle sinks) and its no surprise that dude tired out and sank.
Also, open water swimming is a hell of a lot harder than lap swimming, because you have to lift your head to see what's in front of you and make sure you are swimming in a straight line. I'm betting what little swim training that guy did was in a lap pool.
edit to add: Looking at the video again, the first three out of the water, especially the first guy, had decent technique: Head up every couple of strokes to sight ahead then back down, long strokes, arms out of the water on the recovery part of the stroke, horizontal body position.
Drowned dude: vertical body position, head up the entire time, arms dragging through the water on the recovery. Like I said before-- textbook distressed swimmer.