I am curious as to how Stack Overflow is perceived as toxic.

Name calling of "borderline autistic" and "absolute jerk" would be met with fairly quick community moderation and condemnation there or on Wikipedia. Meanwhile, calling someone autistic as either a pejorative or an insult here (or reddit) is seemingly acceptable and is being defended.

Can you not use autistic as an insult, please?

Dude, I don't. My 5 year old son is in the spectrum and I probably would be diagnosed as well by todays standard. We are lucky enough to be on the high-functional end of the spectrum but it is still taxing on the family. I can't possibly imagine how worst it is for more severe cases and I'm sorry if I offended you but many of my best friends are also borderline autistic and they are the best people I know.

Perception of insult is highly subjective.

Autistic and borderline are barely perceived as positive words in our society so I can follow the argument of insult when you describe wikipedia authors as borderline and autistic. Although I see that you did not mean to do harm with your sociological background.

It doesn't sound like an insult - it sounds just descriptive.