People drink in the passenger compartments of limousines. Some liquors are flammable. Not just Everclear. Most brandy, Grand Marnier, cognac, others.
Vehicle interiors don’t just ignite, there was something that served as an accelerant. Freebasing has been mentioned, too. Would people freebasing in the back of a limousine be more or less inclined to be drinking Couvoisier, and would they even notice at first, if they’d lit something on fire unintentionally?
The other thread suggests the fire started in the trunk. I’m guessing multi-kilowatt stereo system cooked off.