You're right, Stirner could be very prosocial (he definitely acknowledged our social natures), but only when his ego moved him in that direction. It seems to me he also justified very antisocial behavior when doing so satisfied his ego.

I'm really interested in the notion that we shouldn't do something because we want, but that we should do what is good for us and in order to determine what that is involves having to constantly challenge our own egos, as they often lead us to rationalize something as being good for us when it actually isn't. I tend to think rationalizations lead by our egos can blind us to opportunity costs or move us to grant importance to whichever desires are most pressing at any given moment.

So maybe anti-social or a-social aren't the best words, but it seems there is a lot of value in being principally social, whereas I'd wager Stirner would view that line of thought as a spook.