I would guess that there are no specific rules for leaky planes other than "use common sense" and "fuel spills should be cleaned up". Calling the fire brigade to oversee the clearing of a fuel spill under the wing of your plane seems common sense to me.
Kerosene is not that flammable and a starting fire can be easily controlled by someone with a hand-held fire extinguisher.
So, the fuel spill does not pose imminent danger to plane and passengers; there is no need for an immediate evacuation. Having passengers walking near active taxi- and runways would be more dangerous for them then holding them in the plane; at least until an orderly disembarkation (with stairs and buses) can take place.