like his only "autistic" trait has just been "being indifferent" to people
Since nothing is officially stated it is very possible you are right. However, as a person with autism, I really relate to him and think that he shows a lot more traits than simply “being indifferent”. Despite this, I respect your opinion as you probably have different experiences with what autism is, especially when in most media autism is portrayed at its most intense (focusing on people with high support needs and not those with low support needs).
Idk tbh he definitely doesn't show cluster C traits, and the only cluster B disorder that could somehow for is the antisocial disorder, but we can see that he doesn't truly lack empathy or remorse as he's shown both. Autism sounds more reasonable in this case, especially since he's a high functioning person and that means he doesn't necessarily mean he's on the end of the autistic spectrum, he's just on that spectrum.
Wow the mistakes... Sorry it's 4 am
but everyone is saying hes autistic when its more likely he has a cluster b/c personality disorder...sangwoo (semantic error) definitely autistic coded