I don't think LIZS was saying, or even implying, that at all, Hak. It is just more likely that parents who are as invested in education enough to pay are more likely to look for, and pay for, a diagnosis if their seemingly bright children are performing more averagely at school.
With regard to the OP, I do remember a few years ago, a post of the TES website from a teacher in an expensive and prestigious girls' school who said that 50% of her year 10s and 11s had a diagnosis of dyslexia and had special arrangements.