Its a fair question,even with the lp valve shut (if it has indeed shut properly,sometimes they dont) there is still a surprising amount of fuel in the plumbing,also if the cowls were ruptured then discharging the extinguisher means a lot will blow out of the gaps and not be contained under the cowls,smothering the fire.
Hence why lp fuel valve ops checks and cowling fire seals are scheduled inspections.