thats not true, there are TWO ways to become an Espada. To be particularly strong. Or to be particularly useful/unique. Its quite obvious that he is physically weak but probably has talents (like the Anti-Ban-Kai Room) that make him worthy of his rank.
That hasn't really been stated, if they're just weak I doubt that they would have been selected.
According to the second spoiler, it seems that Grantz is better than Renji even when he hasn't released.