But more to the point...I used to be a crew chief on the 737's big sister, the KC-145, and when I put my bird in Fuel shop, we would have a fuel truck come over and drain all the tanks. Then I'd go under the aircraft and drain the remaining fuel out of the tanks (sometimes quite a bit) with a fuel bowser. Then, when we got into fuel shop, the fuel tank guys would wear respirators and high boots while they worked because there would be pools of jet fuel at the bottom of the tanks, hiding in the recesses that couldn't be drained by the manual drain valve I had used. Not much (probaly less than 20 gallons in the whole plane) but plenty for starting a fire, and that was after I spent an hour doing a manual drain job.
Trust me, there was plenty of fuel in those tanks to start a fire with, and plenty of fumes, too.
Good point, and the fumes are much more volatile especially after being sprayed over the landscape by impact!