They did have explosives in there with them, at least in the form of firework mortars.

Explosives and fuel make a good fireball and fire. Fireworks being a 2 stage explosive, they may have expected the initial bang to disburse the fuel and the second to detonate it as a fuel/air explosive which would be very damaging, enough to take down the building.

Obviously were that their goal they did not sufficiently test. The vehicle contained the initial explosion and all they got was a fire