He sank....
But, they might have meant he was face down? BCs are not life vest. If you pass out, you can still go face down even at the surface. A good buddy in the water would have helped.
It reads to me like he was face down. It doesn't say they jumped in to get him (which would be required if he sank). Anyway, if he was face down he had enough lift to stay on the surface, which would exclude weight as a contributor.