The idea of lighting the drone on fire is really more of a joke, just because it isn't a feasible method, as you said, it would change state to a gas really quickly at high speed and altitude.
Some people are suggesting it could be to upset the sensors in the piston engine, the drone is through the cloud so fast that it is pulling in fresh air almost immediately, also, the MQ-( does not use a Piston engine, it is a TurboProp, which is a gas turbine engine.. basically.. it IS a jet engine but instead of producing thrust, it uses the rotation of the turbines to drive the propellor, allowing for a much smaller engine to drive a prop, which means the drone is more efficient an capable of flying at slow speeds, such as those speeds best for taking pictures/video. Pouring some jet fuel into a jet engine isn't gonna do much.
It is possible that they were trying to block communication, by disrupting the antenna direction, this is why the feed cuts out when the drone is buffeted, the antennas need a perfect line of sight. So perhaps the fuel cloud can disrupt communication just long enough to cause it to crash.
However.. I think it was more of a visual warning, saying these pilots are acting all kinds of crazy, and we best get out of here before we lose a $30 million dollar drone..