If the older parents hypothesis was not true, you would not find higher autism rate is younger siblings or how do you account for that in your alternative explanation?

But sure, as they say in the paper, its most likely not a single large effect variant that causes autism but usually the combination of multiple rare variants. The genetically inherited part may also play a rule. For a complex disease such as this, its not a single cause that is responsible even for one patient. A tall basketball player can be tall due to some luck, tall parents and good nutrition all at the same time.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4927579/ (Autism As a Disorder of High Intelligence [2016])

> The hypothesis that autism risk in offspring is positively associated with high parental intelligence, and high socioeconomic status, traces to Kanner (1943, p. 248), who stated, referring to autistic children, that “they all come of highly intelligent families,” at high levels of educational, socioeconomic and occupational achievement (Kanner and Lesser, 1958; Rimland, 1964). King (1975) reviewed a set of demographic studies motivated by these findings, and reported strong support for the pattern of high socioeconomic status linked with autism, including support from studies (e.g., Lotter, 1966, 1967) that checked all young children (of 8–10 years) in a given geographic area for infantile autism, and thus should be largely independent of confounding ascertainment or help-seeking biases, variation in access to relevant health care, or variation in parental awareness.

Note that these observations are 60+ years old. Read the entire "Socioeconomic status" section of the paper I cite. They argue the same and state genetic testing should be done to back it up. These seems an ideal study for 23andme (yes, yes, I know how a vocal minority feels about this org).