Not defending the policy (it's indefensible) but as per that article there was a single case 20 years ago of a rejection because of Autism which was overturned. So I don't think it's fair to say Australia doesn't allow people with Autism diagnoses to immigrate.
There are rules (enforced) and there are rules (unenforced). This is still a bad rule, and needs to be fixed. But we can still acknowledge that they're doing a good thing by ignoring it.