It is not difficult to aim up or down. In fact, the greater the angle from horizontal the less the bullet trajectory is affected by gravity. It literally shoots straighter. Well trained shooters will understand this and compensate for it easily. That video talks about air rifles, the ballistics of which are not relevant to aiming the weapons being used to shoot at drones.

The real issue is that it is very difficult to judge the distance and speed of objects in the sky as there are few objects to gauge the target's distance against. Let alone that the shooter may have no idea the actual size of the craft they are trying to shoot down.