“You wouldn’t self diagnose cancer, so why would you self diagnose autism?”
This is such a ridiculous comparison that it should be obvious satire, but these people are serious.
Cancer is a life-threatening illness. If someone has cancer, they’re going to want medical care so they can cure it or at least survive it. Without the medical care, they’d most likely die, even if 100% of society accepted them.
Autism is simply a harmless difference. Everyone has some things they’re good at and some things they’re bad at. Autism just makes it so the things that someone’s good at or bad at are not within the bounds of what’s socially acceptable. Some autistic people want professional help, while others can function just fine if they simply understand themselves better. Autism is not inherently life-threatening and does not need to be cured.