Thunderstorms are dangerous. They come up quick and can produce 7+ foot chop and rogues which will breach 4’ of freeboard. Only a full cuddy with 4’ bow freeboard won’t get swamped in that. And you have to remain bow-into it to avoid rolling or stern swamping.
Their boat is open bow with less than 3’ bow freeboard. I.e., they swamped, gunned the engine, got broke up on successive waves and rolled. IMO if they were not wearing their life jackets before that point they are not wearing them. Being ejected in that type of seas there is no chance of successfully getting back to the boat to secure equipment or hold on. It’s 3500 lbs of waterlogged boat pitching 14’ up and down, left and right, forward and back. It’d knock them out if they tried.
Sadly I think you've hit on the most probable scenario.