I first saw the term middlebrow in that thread, and I'm still not sure how i feel about the term, but "lowbrow" refers to completely non-intellectual things, and would not appeal at all to someone that values intellectual thinking. I hadn't heard the term until that comment thread, but apparently it has its own Wikipedia page, and has been around since 1925.

Because middlebrow posts appear intellectual on their surface. They are not quite lowbrow. But they aren't highbrow either, for the reasons explained.