I genuinely want to understand: What defines someone as autistic (a condition centered around significant social impairment) if that person has friends and/or is able to make and keep friends?

To avoid getting into complex neurobiology (to a level that is no doubt beyond what I have a right to describe, anyway):

ASD has to do with how our bodies initially parse incoming sensory (or sensory and notional) data. Neurologically, we hear things more intensely, we see things more intensely, we feel with touch and taste with nose and tongue more intensely, and we do not start out with the same degree of desensitization and selective awareness that neurotypicals begin with. (It's more complex than that, of course, but I'm just giving a basic sense.)

So what they can do as a biological inheritance, we have to learn how to do with cognitive training and behavior. And this takes effort, so we can not sustain it all the time, whereas for them it is cost-free.

If the complexities of sensory information of the world were a vast sea, then neurotypicals are born with gills, and we had to learn how to hold our breath. So while they can hang out underwater all day long, we can hang out only so long before we have to surface and catch our breath outside that sensory information sea.

Those of us who are high-functioning autistics have learned how to hold our breath a long time. Some people can only hold it for a short while. And some people have biological problems that mean they may never develop the ability to hold their breaths at all.

All three types have come online here in this reddit forum. However, I began this thread in part because a small but very vocal group of ASD trolls have convinced themselves they can not ever hold their breaths a long time and then attack those of us who can. The "no true Scotsman" fallacy is very, very popular here with a certain crowd. I have had more than one person here tell me (in private post or public? I don't remember) that the fact that I speak well or have friends or have a doctorate means I "don't count as a real autistic"; one person here once referred to me as a "freak" and told me I had no right to participate in this forum if I am high-functioning enough to have friends or can look people in the eye.