I think your point is quite insightful. One way to get an edge over other high school coaches is to do some college style training stuff that other high school coaches either don't know how to do, or because of numbers and safety reasons, err on the side of caution and choose not to do. High School athletes in such a program get an edge during high school, but often don't make as much improvement in college because they've already got the improvement in performance from employing those techniques.
If a high school coach does this and consistently wins in high school, he or she is a pretty good coach. If a lot of his or her athletes get hurt in high school, he or she is not a very good coach.
If the standard for being a great coach is not developing kids closer to their potential than other high school coaches (so that they make major improvements in college) yet still developing them to the point that they consistently beat other high school athletes, I doubt there are many "great" high school coaches.