UV light tends to kill fungi, so it seems like they are stressing the fungus and then trying to condition the signals they get from the fungus to create a directed motion. Essentially using the fungus as a UV detector and simple form of logic. They did not, for example, let the fungus vehicles wander randomly, seeking moisture and better soil to see what the fungus desired, though that might be an eventual goal, a sort of environmental sensor. That's my guess.