That warhead has both a proximity and contact fuse, the destruction of the jet would come as the structural failure was acted upon by the aerodynamic force of the 500kts. Structural failure at altitude would account for the no call on guard, the disappearance on the radar, which I think is overblown and the fact they haven’t located debris from the plane yet. Add that it went down in a sewer full of flotsam and jetsam anyway, I’m not surprised they haven’t found jack yet.
One ‘flight depiction’ has it ‘disappearing’ near Hainan Island instead of SW of Viet Nam. In which case, they are 800 miles off.
Black boxes should be pinging away though.