Let me state, before going any further, that the following is based on no expertise or in depth knowledge of the subject.
However, when someone asked earlier in the thread 'what has changed over the last couple of decades' I would state that it is probably worthwhile considering the factors that children are exposed to in their infancy.
Particularly, the amount of extremely young children who are consumed by screens - before they can hold a conversation in a lot of cases.
I don't think that this 'causes' autism, as it is obvious, if you have ever met anyone with a significant level of it, that it is very consuming and unlikely to be something that is 'caused' by one or two things in particular.
However, the question is what has led to a lot more diagnoses of it, including a lot of 'mild' cases.
It is a fact that there is a developing trend for the mental health of young people as a whole is deteriorating. Mental Health issues in young age groups are rising hugely. It is a very worrying trend. There is also, according to many, a significant rise in young people experiencing a lack of resilience and grit and struggle to deal with pitfalls in life. This may or may not be the same thing.
It is also true to say that a symptom of some autism is liking to be in your own world, wanting to be shut off and not liking change, interruptions or disruption. Something that is actually a very common description of children playing with screens.
Is it not worth considering whether the rise of diagnosis of 'mild' autism could actually be doctors mistaking the symptoms of a growing decline in childhood mental health and putting it all under the easy label of autism? Obviously, unmistakable, severe autism could not be mistaken in this way but I would imagine that the symptoms of 'mild autism' and the symptoms of the increasing amount of children with mental health issues could be quite similar.