The point is that you can send lots of tiny drones to converge on the turret. They have to cover 1.5km (though that's only the distance if there's no cover, i.e. the gun is on a perfectly flat plain with no trees, buildings, etc). If the drones are travelling at 36m/s then that's about 50 seconds to cover 1.5km. If you drop 2000 drones, you'd need to kill 4 each second. That seems like a lot.