Autism is not a MH on its own, but around a third of those who have it also have learning disabilities. Those who have Autism are also much more likely to have severe associated mental health conditions.
A proportion of my caseload as a disability outreach worker was supporting families who were dealing with autistic adult children, many of whom were violent as a result of these conditions. Help for these families is patchy and in some areas non-existent so, as seems to have been the case with this girls’ father, they are left to struggle. So you can’t really state that prison is the right place for offenders who have these conditions without examining all the factors.
The state of MH services in the UK means that too often those with MH conditions don’t get appropriate intervention before they offend and they end up in prison instead of appropriately hospitalised.