One of the worst habits distinctive to online discussion-board writing (especially the sorts of places with lots and lots of people and where it's fairly hard to get permanently kicked out—like here) is too much hedging and over-specifying to try to head off shitposting by bad or bad-faith readers. It's all over forum posts, and it's poor writing, but without moderation that slaps down responses based on plain mis-reading you have to write that way, or your post will spawn all kinds of really stupid tangent strings of posts (and they still do anyway, sometimes). And, yes, the excessive and too-close-together repetitiveness you mention is part of that.

The result is that a ton of web forum/social-media posting would, in any other context, be fairly poor writing (even if it's otherwise got no problems) simply because of the the extra crap and contortions required to minimize garbage posts by poor readers who are, themselves, allowed to post to the same medium.

This is in addition to, though not wholly separate from, the tendency toward combativeness in online posting.

I totally agree with this. I would add that it's well beyond the discussion boards. It's probably most clear there and it's well possible we learned it there and then took it into our social interactions everywhere, but the majority of my irl interactions—except with my closest friends—are sorta like this. Sometimes I think its ADHD, other times I think it could be any number of things, but I think to say anything that isn't dead simple (or in dead agreement with the other person), you need a few sentences. Often, you need to hear the third sentence before the first will make sense. But if you get distracted by the first one or can't suspend your disagreement enough to get to the third you will think the person is mistaken. You'll think that about both their first point and the larger one, which you didn't really hear or even get to but thought you did. So the speaker does the hedging each sentence in hopes of getting to the third (or whatever) sentence.

Despite being a different kind of writing, there are some interesting parallels with the article in what you wrote here