SinisterMexis isn't actually a bronze level player, it's just that he hasn't played enough casual ladder games for his ladder rating to catch up to his actual rating. He's a low silver level player.

You're correct, but due to the lower, significantly better on average population of the competitive ladder, his MM rating is only top 30%, which is still subgiant.

SinisterMexis isn't even the highest rated player in this category; JayBird is another bronze/subgiant player who's quite good and while I haven't seen xxNihaalxx play he's the highest rated player in this category (of players who are currently on the ladder). Both of them are much lower in casual rating than they are in competitive rating so I suspect they're simply much better at 1v1s than they are at team play.