My best friend's son, who I watched daily for two years and who has been a close part of my life, as my son's best friend, for four years, is autistic and he was in school, undiagnosed, until third grade. Many of his teachers refused to believe he was autistic and not just "weird" because he performs acceptably in class.
He is also off-the-charts brilliant, and extremely intelligent high-functioning autistic children are harder to diagnose because, while doing very poorly
for them, they tend to function at a level that's closer to a "normal" child of their age. Highly intelligent high-functioning autistics are finally being recognized and diagnosed, when often they weren't before.
Also, I think you should watch who you're accusing of never having lived with an autistic child.
My youngest is still in assessment at OHSU, but despite being highly intelligent and doing well in school, she has some clear neurodevelopment issues and a language delay, and has had three-hour screaming fits because it was getting dark and she didn't want it to get dark. Another good one was her hours-long screaming, choking, sobbing meltdown because she wanted me to get all of the water out of the house.