I wouldn't agree that a lack of autism diagnosis is definitive because that diagnosis is primarily based on need for treatment. (Notice how the diagnostic criteria are exclusively variations of "finds it difficult to be normal".) I only see the tokenized / scratch-blocks writing style with Aspies. (This is likely what comes off as LLM-ish to others.) Non-autistics tend to be very sloppy/imprecise in comparison. ADHD primarily has to do with behavior (misbehavior in fact) and so wouldn't solely be responsible.

I have a few friends who are definitely at other places on the spectrum and yet not diagnosed. The funny thing is Aspies are the most likely to be diagnosed because their early development can be most obvious, but it's still not always caught. (This is supported by recent research identifying the presence of a finite number of distinct genetic phenotypes in the autistic spectrum.)