4chan means narcissist when they say autistic

In most contexts.

Recall Ghost in the Shell. Autistic mode is when they turn off their bluetooth network, aka airplane mode. This is a correct use of the Latin. Narcissists necessarily function in this kind of autistic mode all the time - narcissists have a version of closed shell syndrome. 

Regular autists merely have difficulty understanding others because we're so different. The Caino hypcriens brain functions by assuming the other brain works more or less like yours does, which is a problem when it doesn't. If the assumption doesn't hold, the function constitutes a bug. Unfortunately allists are actually incapable of debugging the function, so they're just kinda boned when it comes to dealing with autists. Once we grow beyond the larval stage it's up to us whether we want to make the allist comfortable or exploit them or troll them, and they can't do shit about it except go full shrieking monkey mob and lynch us. Even then, it's not hard to spot the line.

For a narcissist it's actually impossible to understand others, because they're entirely invisible. They can't see anything except their own reflection. (Reminder: which, unlike Narcissus, they hate. If it were consistent and not self-destructive we wouldn't call it crazy.)

 

Excepted context: autistic map makers or autistic flag-finders are genuinely autistic. Or more probably spergs. Spegs have in fact a heightened capacity for empathy, but it tends to be object-directed instead of person-directed. Still, as the global capture-the-flag incident showed, spergy empathy will in fact reveal more about the person than allist empathy, in the end. 

This is part of the reason allists hate spergs so much. This is the reason narcissists immediately go into an incandescent berzerk fury on contact with spergs: their illusionary fake persona will be casually pierced. Autists will straight-up fail to notice the narcissist demanding they play along, whereas the allist can be fooled into playing along even if they resist.