Surely it wasn't the deploying of "foam and water" that led to reduced cover.

If a fire tender and its associated staff has been sent to a first potential incident, then they are out of place, and "occupied", even before they get to the point of deploying "foam and water" (if it ever turns out that is necessary).

If another incident occurs somewhere else on the airfield - the fact that they haven't started spraying on the first potential incident in no way allows them to magically (and simultaneously!) attend to a second potential incident!