I always hesitate yelling "hack", especially with COD4 & 5 because of a few "perks" which used to be hacks in the old days.
First is UAV. A good player can take a quick glance and (knowing the map) have an excellent positioning of the "enemy" thru trees and walls.
Second is "deep impact" perk which allows bullets to go right through "thin walls", bushes, etc.
Third is the terrible spawning...notably on the map in the video. A small map, and good players know where the spawns are very quickly. I speced a player with similar "tactics" as shown. He "knew" where to aim because of the spawns...right through trees while he was hidden in the grass. I speced both 3rd and first person (try 1st person as it gives you a much better perspective than the 3rd person view) and couldn't "swear without a doubt" whether he was good or whether he was hacking.
I am not defending the player in the video....however I know this game requires a much more subjective view/analysis than the "normal" shooter.