Autistic people have relationships and one night stands and everything in between. What he's thinking of is the word "loser". Don't conflate the two.
I think he meant they were going to think he was autistic because of his single-minded obsession with his shrimp. Also a bad stereotype, but a different one, at least.
This is 4chan... 4channers say Autistic to mean antisocial losers or actually autistic. No this isn't cool, but it's also important to understand there's a lot of edgey and cringy losers in 4chan that use a lot of slurs by default.
Fake and wholesome