Tbh it's a bit odd to assume they're on the spectrum because they're a loner.
If they are against dating at work to avoid awkward situations, perhaps describing hanging out as a non-date (😉), bringing up your "seeds of romance" in another comment, and saying how cute they are is setting yourself up for disappointment. It's giving mixed messages.
It's not passive aggression to point out that you're not being cheeky calling it a "non-date" instead of a hang out - a hang out where they made it clear enough they don't have an interest in work relationships. If I found out someone was describing a platonic hang out with me as a non date and talking about seeing where things go, I'd get the ick.
Your post gave me the ick on their behalf, it wasn't because I'm profile scrolling lol. I saw your first comment before even getting to this one because I read the mega sorted by new.