I would bet that most, if not all, of the raiders in the top world guilds have spent thousands of hours working at their game to get to the level that they are, and I would be most surprised if they were all considered "wow-geniuses" at the start.
As an example, straight out of school, only about 5% of people might have the aptitude to start training as an engineer. That is not to say that the other 95% could not be trained, with sufficient time. It's generally simply not worth it.
Best in the world people are exceptional. They have a special talent, and combined with enough effort are unbeatable. But there is very little stopping any ordinary person from becoming almost as good (better than 99% of other people), provided they have enough drive to put in the required effort for a sustained period of many years.