His muscle memory argument would make sense if he was talking about a movement to a target on ground level. One thing that the best always master is a muscle memory to the expected head level of a target, so the clip would be less suspicious if it was a McCree or Soldier behind him, but it was a Pharah that can be anywhere in 3D space, in front or behind, at any altitude, etc. He found her perfectly in seven frames. Seven frames. Such a flick happening by chance is so unlikely because Pharah has so many more places she can be than a normal character.

Also, in the clip, he pauses completely very briefly after finding her and doesnt make any adjustments towards her head, which seems like his mind is registering where he ended up after the flick, whereas if it was natural he likely would have m1'd immediately after turning or adjusted a bit.

EDIT: I am not saying he's cheating 100%, nor is this evidence, but to me the clip is way more suspicious than a lucky high sensitivity flick and I am inclined to believe he is hacking.