Heritable != genetic.

There are very strong reasons to suspect that gut microbiome at least plays a major role in at least a large portion of autism cases, complete with studies where we've been able to induce/reverse a subset of autism symptoms in mice, and even humans, as well as linking individual gut microbe metabolites to them. And ~80% of autistic people have gut problems.

There's no question in my mind that at least some of the propensity to develop autism is genetic, but calling the whole of it genetic feels ridiculous in the face of preliminary evidence to the contrary consistent with the massive increase in diagnosis rates beyond what can be explained by changing criteria.