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After looking at the video, I’m assuming you are talking about the sniper kill. Easiest way to explain it is that the bullet magnetism in Halo 5 (Sniper Rifle in particular) is rather strong.
Also, if this person recorded the game in theater, it glitches and makes it look like the shot is not spot on.
I agree that the theater glitches but if you search on the internet 343 decided to increace bullet magnetism in Halo 4. So i assume that this Halo has the same bullet magnetism like Halo 4. Even the MCC has more aim assist and bullet magnetism than the original games.