They don't need to be evenly distributed to be strong evidence against the Neanderthal connection. If autism was basically "latent Neanderthal ancestry", then you'd expect essentially no autism outside of people with European or Middle Eastern ancestry.

And it seems that the prevalence of autism is actually fairly even, but might actually be lower among white people.

https://www.cdc.gov/ncbddd/autism/addm-community-report/spot...