The residue/dust in CS gas is very flammable. That coupled with the strategic punching of the wholes(for a wind draft) when they pumped in the gas, as the movie states,
the compound became a tinder box.
When the fed shot in the flash bangs on the windward side of the compound, the place burned to the ground in minutes...
CS is not as flammable as gasoline, but it was an accelerent none the less.
I haven't seen this film but in past accounts it has been stated the Davidian's had many containers of gasoline in the compound to burn it in case of an attack. I would tend to believe gasoline was the accelerant.