I don't think it's a reading comprehension problem, it's some sort of cultural problem. These people are reading what we are typing, but that's not what they want to talk about. So they will take anything, even tangentially related and disprove a component of it so they can reframe the conversation back to what they wanted to argue about.

I actually find myself doing similar things. Essentially I will write out a long winded comment, then realize that the person I am replying to has nothing to do with what I wanted to say. Instead I was paraphrasing all of the comments, coming up with a point I wanted to make and then ramming it round peg square hole style into someone else's comment tree. I have been deleting a lot of comments before even hitting the post button in the last 6 months or so since I realized I was doing it.

TLDR: A lot of people online are not arguing in good faith.