Well past conflicts have shown Soviet and Russian pilots to be fairly skilled in the area of air to air gunnery, and whilst certainly Russian air force combat training has suffered since 1991 (although it is now improving) a drone with a fixed speed and path would be a very easy target to shoot down with a cannon. Its more likely that Russia wanted the video to demonstrate Russian air to air missile technology or that it was simply a good chance for some missile testing.

First learn about air combat when post. UAV is a slow target. If you are to shoot it with your cannon it turns to a numerous debris to easily damage attacking fighter. Secong it still a brilliant shot since UAV is a harder target for AAM than any stealth bomber due to it's size and composites it made from.