There is a huge difference between a bouyancy aid and a life jacket. A life jacket may have provided enough bouyancy to get two people to shore, but a bouyancy aid would almost certainly not. Life jackets tend to have crotch straps to stop them pulling over uncounscious peoples heads and are also designed to turn people onto their back, bouyancy aids do not.
I’d hazard a guess that the adults werent wearing bouyancy aids or life jackets, and that the boat didnt have bouyancy bags in it. I’d also hazard a guess that the guy who swam to shore thought that it was his best course of action and probably his only way of surviving.
Hypothermia and shock kick in very quickly in cold water. A boat sinking without trace would add to that.