I call bullshit on the excuse that he needs to do it as an autistic stim. Even if that were true, it's possible to highlight text temporarily just for himself. Clicking the highlighter icon so it's highlighted for everyone does not help him read it. And as you pointed out, it's unlikely he has the time to read every article he highlights. He's spamming, plain and simple. He is highlighting everything in order to get his name out there as much as possible and get followers. Excessive highlighting is clearly against Medium's rules. If he was spamming us with affiliate links in the comments and claiming he needed to do that because of his autism, we wouldn't be tolerating it.

When you blocked him, did you also report him?