They are spraying the areas where it touched down and sparked. If it leaked fuel on impact and if it were to catch fire, you do not take the time or risk your life to walk over, check for moisture, dip your finger in it, and give it a sniff. You just spray the hell out of it, just in case. Plus you also wash off any shrapnel that may be a danger to other planes.
It should be noted that the truck that came in from the right did begin praying the plane just before the video ended. Spray the landing path/potential fuel trail and spray the smoking hot plane.