Conveniently it's a single tree that catches fire with none nearby. Plus the drone is mere minutes away. What if it's densely packed brush and the drone is further away? Or a powerline touches off the fire setting hundreds of trees aflame? Or multiple lightning strikes touches off many fires in a wide area? This thing looks as if will be very expensive and work only some of the time.
I don't think this would even be able to put out a single tree catching fire. Generally the fire is too vertical and the extinguisher would land on the ground and not be activated by the heat, so it won't go off until flaming branches fall down, at which point the fire is too large.
You’re referring to a canopy or crown fire. It depends on conditions, of course, but it usually takes time for enough heat to build up that a fire crowns. There’s generally a period where it will burn through the lower and mid stories and it spreads.