That is an odd leading theory.

Afaik the leading theory is people are born with it, and "it" can show in a lot of different ways, but a list of symptoms are very telling and there are clear patterns of behavior associated with it. Also there's a specific age when people start to notice autism in children most prominently.

From what I can tell, you can't "become" autistic.

And vaccines and autism only overlap because the age where autism tends to stick out in children falls bang in the middle of the age range where children get their vaccinations. So it lines up. But in a bunch of studies a direct correlation was never found, and neither was causation.

The leading theory for the cause is (and bear in mind I'm simplifying a heckton here) "we don't know". There's possibly a genetic component involved, but if so, then it's super complex and doesn't have a clear causal mechanism (in that a person could have most/all of the genes of interest for autism, and not be autistic, and we don't really know why that is. Or why some people have none of those markers, but still have autism). Environmental contamination is a possibility, but we don't know nearly enough to guess what that would be, given that it wouldn't necessarily be microplastics

But there definitely is a lot of evidence supporting OP's claim, in that we are very confident that our increased understanding of autism has led to an increase in diagnoses (especially amongst groups such as girls; I am one of those girls who only got a diagnosis because of our shifts in understanding). We are extremely confident that our rate of false negatives has dropped over the years (i.e. people who actually do have autism but go undiagnosed). However, it's hard to estimate whether the actual underlying true rate of autism has actually gone up, given how much our understanding has changed in a relatively short span