Affluence and Autism: Cause and Effect?

Have you seen this new University of Wisconsin study that correlates an increased prevalence of autism with greater household affluence?
This isn't the first study to reach that conclusion. But what does it mean? Many researchers dismiss research like this by saying wealthier people have more resources to get an autism diagnosis. They say more educated people are more likely to pick up subtle differences in their kids. And perhaps they're right.
Does that account for all the difference?
The incidence of autism combined with intellectual disability is not strongly (1.3 to 1) correlated with affluence. It's only the less severe forms of autism that are more common in wealthier homes. Is that because autism combined with ID is obvious, but the less severe condition is not?
Maybe . . . but maybe not . . .