Well, think of it this way: I have a 6-inch 360. Some people have 3, some people have 9. Both are ridiculous to me, because that isn't what I am used to, and I see them as extremes on the spectrum. I'll still recommend lower sens closer to mine to people newer than I am, but that's because there's a general false notion that a higher sensitivity will improve your aim by itself, and I want to dissuade that and encourage them to start at a medium and play with things from there.
He was very good, believe it or not. He's just that sort of person, who has a knack for picking things up quickly and efficiently. He learned Soldier and Demo at my request to play for our 6v6 team when it was around, and was probably high open in a comparatively short amount of time.
It isn't that common a thing, sure, but it isn't unreasonable to believe. I met a 13-year-old who had 200 hours logged (I paid for his premium) and who was also probably high open out of the gate with Scout.