Some, if not most, top diamond players are pretty stupid and are just really really good at a single strat like 4 gating because the build order is just second nature to them(day9 talks about one of the top players in the world who emailed him lolzing about how bad he is and how far he rode into diamond on 4gate; idk how true it is, but it's certainly possible). These players don't know a single transition, what units are cost effective against what, any neat tactics, but they do know a pretty good build order that has a powerful timing push and because they play every single game against every single race the same way they can pull it off pretty well.
Playing the game takes a little bit of good decision making, a little bit of experience with when popular timing attacks hit, and a lot of muscle memory shit. The latter two require a lot of playtime instead of intelligence.