@quentinc:
I don't see how you arrived at that surmise ... are you supposing that because damage is extensive, the origin of the fire is not small or simple?
It's in the nature of fire that small and mundane origins (e.g., a cigarette) can lead to vast destruction
Indeed - if the aircraft had no-one on board then the fire would not have been detected until it burnt through the outer skin of the fuselage (and with no steps and no airbridge then this is pretty much the only way the fire would have become known about as nobody going on board when it's "ready for towing" could have discovered it as there's no access for them) which means the fire probably had quite some time for the fire to propogate internally prior to discovery.
After all it takes almost no time at all for a fire to propogate and develop
Note in that video the fire is ignited at 00:40. By 1.10 (30s after ignition) the bed and table are already well ablaze (you can see the fire on the underside of the bed) and that by 1.25 (45s after ignition) the room has already reached flashover.
If anything, in this situation, the low burn temp of the Composites and lack of fire insulation has saved the aircraft from more extensive damage as it caused the fire to become visible much sooner (and therefore with less internal damage) whereas a metal skin with insulation would have contained the fire leading to much more extensive internal damage prior to the fire becoming externally visible.