Comment by BirdieNZ

I don't think that's the conclusion of the paper you linked, although there are papers suggesting that ADHD and autism are essentially subtypes of a single overarching disorder.

The more common view is that because prevalence of attentional deficit in autism is so high, it's rare to find someone with autism who isn't also able to fit an ADHD diagnosis. This might mean a shared common ancestor as it were, or perhaps autism has attentional deficit as an intrinsic part of the disorder rather than people with autism also often having ADHD.

The other direction is the amount of people who have ADHD who also have autism. In this case, while the overlap is higher than the general population, it is easy to find people with ADHD who do not meet the diagnostic criteria for autism.

Put more simply, people with ADHD aren't necessarily autistic, but people with autism are usually ADHD.