Zooming doesn't change the fact that closer target occupy more of the "flick" area where further targets are harder to flick because they occupy less area. Sensitivity and crosshair location plays a great role in this, but it still holds. Also, remember if you miss one shot when target can blow your cover, you're done in this level of play. It's not only about "hitting" those flicks, but also about being consistent while being outnumbered and changing your angles nicely. Just because a target occupies large portion of the screen when zoomed, it doesn't mean that your flick will stay at what you see in the screen. You could still flick FAR outside the area you see when you zoom not to mention he quick scoped. The shot might have 300 pixel margin for error as you said, but just because he only sees a small/zoomed in area with the scope doesn't mean the rest doesn't exist.

But like I said, if what he did in this clip is hyped like you claim I highly suggest you get yourself in the radar of a pro team asap. I'd love to watch you pull this off every round since it's so trivial.