It’s most likely gasoline. It’s very difficult to engineer upholstery and rubber to be resistant to prolonged exposure to an open gas fire. Usually the best you can do is get to a minimum safe time for certain temperatures.

The highest standards you’ll run into day to day are baby clothing, bedding, and residential wall insulation.
The reasons for those being specifically regulated should be relatively obvious, and are respectively heartbreaking, scary, and sensible.

Cars tend to be going fast when they encounter issues, and there’s a lot less ability to make a lot of assurances. As a result, cars tend to be designed for controlled failure rather than resilience. This allows to car to fail around the passengers, hopefully resulting in the car, which is totaled anyway, absorbing the damage the passengers would have otherwise gotten.
We can make a car that can take a 45mph collision with an oak tree. We just don’t know upfront that that’s how it’s going to crash, and the squishy people inside can’t be made to tolerate a 45mph collision with the dashboard. So instead of making a perfect fuel tank, we just make sure that if it breaks it tries to rupture the fuel away from the passenger compartment. Instead of making the upholstery incapable of burning (which comes with downsides like “expensive”, “uncomfortable”, “ugly”, “smelly”, or “even more toxic than current flame retardants”) we make it able to resist burning for as long as it would take for the air inside the vehicle to become deadly hot. It doesn’t matter if the seat fabric is unscathed if the fire is hot enough to warp the metal.

Beyond all that, Tesla’s are notoriously poorly engineered, and in that category the cyber truck is best in class. I do not know, but would not be surprised, if accelerant was simply able to seep into the more flammable parts of the car from the outside.

As for surveillance catching the people, covering your face, obscuring identifying marks, and simply being far away by the time anyone notices the fire is a good bet. The police might try a bit harder because it’s an expensive property crime, but it’s ultimately a property crime where no one is going to be building their career on it, so there won’t be real incentive to go above and beyond.