It wasn't until 1980 that the main guide for mental illnesses, the Diagnostic and Statistical Manual of Mental Disorders, published the six-point criteria for diagnosing the condition (and none of them were about how crappy your mother was). Finally, kids who once would have been labeled mentally disabled despite high intelligence now had another possible diagnosis. We didn't even start throwing the phrase "autism spectrum" around until the mid-'90s -- meaning doctors on the cutting edge of psychiatry were just waking up to the fact that there's actually a wide range of symptoms of autism at around the same time kids were walking around with one overall strap hanging down and singing Color Me Badd.
Archimedes had his bathtub, Newton had his apple, autism doctors had 90210.
So, yeah, claiming that autism is new is like saying germs didn't exist until we invented the microscope.