People also specialize a lot more now than in the past. If you are in an office full of programmers and all you ever see are other programmers then the chances of you marrying another programmer is higher than if you are in a mixed environment office.
You might have a point about doctors marrying other doctors, but not programmers. That almost never happens, because there's so few women in programming. 90+% of male programmers are not going to marry female programmers because they simply don't exist.
From my experience, programmers and engineers seem to generally marry women who have absolutely nothing to do with tech. The ones who marry "up" the most will marry lawyers or accountants, the rest seem to marry "down" (i.e. a woman who's not a professional, like a secretary or a stay-at-home wife). And a bunch of them seem to stay perpetually single, because women in this culture generally despise men like this.
I believe age of parents is a risk factor. The other big risk factor is if you have autistic traits yourself. If you have autistic traits then your kids are more likely to have those traits and also more likely to have full blown autism.
Now this I can't disagree with in any way. And perhaps more specialization and people marrying within their profession has a little to do with it, but personally I think age has more. And since professionals/"high IQ" type people also tend to be the people who have kids at older ages, compared to less-educated people, there's going to be a big correlation here.