I know the thread has moved on a bit, but I do wonder if the autism 'vulnerability' is actually a red herring Autism is a very broad umbrella, as I'm sure you know, but around 25% of people are non-verbal and a slightly larger number will be verbal but with severe learning needs.
I work with a lot of children and young people with severe and profound and multiple learning difficulties. They often have autism as well (60-70% at the last count). These children's learning needs and autism often relate to a known medical difficulty, either birth injury, prenatal/neonatal infection or a genetic difference. Their exact condition may be rare, described in numbers (e.g. duplication of the long arm of X at position y) or have no name. But autism is readily (and usually appropriately) diagnosed. They are often medically frail. Their actual diagnoses may not be recorded in statistics, but their unifying diagnosis of autism will be.
With regards to learning disabilities more generally, there is the same crossover with medical vulnerabilities. Children and adults with learning disabilities have lower life expectancies on average, with circulatory and respiratory conditions being the top causes of death, so it makes sense that Covid would be more of a problem for this population.
Both groups are also more likely to live in or attend communal settings (care homes, day centres, schools), to need access to essential services through lockdown and be less able to follow social distancing.
Just a theory, but it would be useful to know more so that we can continue to be more cautious for those who need it and so those with autism without these additional complications are not worrying unduly.