The “special interest” thing is very misunderstood among autistic people, possibly the most misunderstood symptom. I’m not even sure where that term came from. Here’s the DSM-5 diagnostic criteria:
Highly restricted, fixated interests that are abnormal in intensity or focus (e.g., strong attachment to or preoccupation with unusual objects, excessively circumscribed or perseverative interests).
When I was diagnosed, the behavior specialist told me about a little boy who was obsessed with vacuums, knew absolutely everything about them and wouldn’t really talk about anything else. A friend of mine‘s autistic ex-boyfriend is extremely into alchemy - he knows everything about it, practices it, thinks about it nonstop, etc.
Abnormal in intensity or focus is what distinguishes an autistic person’s interest from that of a normal person. And not all autistic people have this trait, which is why I think the term “special interest” is so misused and misunderstood.