I'm guessing you missed the part of it being 800 miles. Please tell me any SA missile with an 800 mile range. I'll buy anyone having one. Where is the detection? Is the radar on Mt Everest? The best SA missile the chinks have a 130# + or - warhead which would obliterate an 500 kt target, unfortunately its range in less than 65 miles. In range there wouldn't be nothing from the target especially considering the aerodynamic forces acting upon a body blasted with 130# of HE.
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.