"Cybertruck’s occupants had been driving back from a 'function' around 3:10 a.m. Wednesday" and "it was unlikely that the Cybertruck’s large lithium-ion battery had caught fire."
Any heavy vehicle with _friction_ brakes can generate enough heat to create fire, it turns out spinning metal is always a stored energy hazard regardless of how the motive power was originally supplied.
Plus upholstery is pretty flammable. They lace it with chemicals to try to retard that tendency but after a certain point they're going to light up and fuel the fire.
Brakes get hot. Hot enough to burn you pretty good even on a car that hasn't been to the track and maybe even hot enough to light organic debris that comes in contact with it on fire. Which is probably pretty difficult normally but might be a lot easier if you bash a bunch of that structure to bits in a high speed accident.
Real mystery though.
The battery pack doesn't need to be compromised to allow this, and even a fault in the LV system will do it. Electrical fires take out ICE vehicles every day.
Most ICE vehicle fires are caused by electrical faults or oil leaks. Oil leaking onto a hot exhaust obviously isn't a factor in an EV, but nothing has really changed with respect to the electrical threat. On modern vehicles, fuel doesn't usually become a factor until the vehicle is well alight.