This thing all started when I noted that BA wrote a lot of passages where people try to analyze their own feelings. And that a passage he wrote for Sakura Kinomoto seemed OOC where it would be in character for Nanoha. And that Nanoha has never seemed like a child to me — even less so than Negi Springfield. Even some of the ridiculously talented grade-schoolers of CLAMP Academy aren’t so hyper focused as she.
When you get someone who is significantly not neurotypical and extremely good at some, ahem, arcane discipline, autism is a reasonable assumption. From there, though, it’s a matter of taste. You framed the question in the Watsonian, but in the Doylist perspective it becomes two questions: did the writers write Nanoha intentionally autistic? And is her character written in such a way that you as a fanfic writer can interpret her as autistic without going OOC? I think the answer to the second question is yes. Even if it’s not canon, there’s value in interpreting characters as like yourself. Just ask Xena fans.
When you get someone who is significantly not neurotypical and extremely good at some, ahem, arcane discipline, autism is a reasonable assumption. From there, though, it’s a matter of taste. You framed the question in the Watsonian, but in the Doylist perspective it becomes two questions: did the writers write Nanoha intentionally autistic? And is her character written in such a way that you as a fanfic writer can interpret her as autistic without going OOC? I think the answer to the second question is yes. Even if it’s not canon, there’s value in interpreting characters as like yourself. Just ask Xena fans.
"Kitto daijoubu da yo." - Sakura Kinomoto