There is no evidence of the HV battery catching fire in any of these photos. In fact the base of both cars is about the only part still in one piece other than the steel sheet metal body and other metal parts. All modern cars have a huge amount of combustible material in them, mostly from plastics, rubber and synthetic materials almost all by products of the petro chemical industry. A short circuit somewhere, even just a small one will create intense heat to melt and then with sufficient heat combust. The burning material melts everything around it and that begins dripping flaming dollops on to everything else around it and the process expands and grows.
As soon as you notice it, it is generally too late even if you do have an extinguisher (dry powder) it will not be enough to quell the flames. There are videos on you tube of fires usually in houses starting from an electric heater or any source too close to curtains or combustible materials and in less than 2 minutes the entire room is engulfed in flames. This is called "Flashover"when the temperature becomes so intense that everything reaches the point where it all catches fire at the same time.
I noticed that the alloy wheels were not completely gone. Aluminium burns ferociously as the superstructure of HMS Sheffield did when hit by an Exocet missile in the Falklands war. The wheels will burn entirely if the heat is high enough but as they are at the bottom they are still recognisable.
If the HV battery suffered a thermal runaway fire the heat would have been so intense that the wheels are unlikely to be there at all afterwards & some other metal parts would be gone or melted in to the ground.