I think it’s some type of mechanical failure myself.
Even a 130lb warhead will not obliterate something as large as a 777. Most air defense warheads are designed to destroy high speed maneuvering targets so the warheads use some type of shrapnel or expanding wire devices. The odds of hitting a small target is very slim. They rely on proximity fuzing instead of direct hits.
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.