The guy is cheating. Go to 1:00 in the video and tell me that that's not the most blatant velocity he's using?
As well as this, take a look at any of the fights they have. You will notice that as soon as he clicks, the hit registers. Now when you're actually PvPing legitimately, you know that this barely will ever happen at the consistency that this guy has it. The reason for this is because first of all, your CPS is not 100% consistent. Whether you're butterfly clicking 20 CPS, or jittering 13, there will always be some stuttering/fluctuating in your hits. Not only that, but aim is a factor in hit speed. If your crosshair is not tracking the player 100% of the time, you will not get combos this fast. Just go onto any practice server, and play a few fights, and listen to the sounds of the hits you get on a player, and it will not be anywhere near as fast as it is in this video.
He's using: Reach, Velocity, Autoclicker, Aimassist (maybe)