There is plenty of introductory material on visual emphasis around, not just covering emphasis in textual media, e.g.: http://daphne.palomar.edu/design/emphasis.html
I agree that line length according to more recent studies seems to be less consistent (some recent studies say speed actually increases with longer lines but there doesn't seem to be any data on comprehension/retention and preference seems to be biased towards shorter lines -- again all of these factors may be cultural). But visual emphasis is a thing -- it's hard-wired into how human visual perception works.
Most techniques for making metadata less emphasized make the metadata objectively less readable. I like having the metadata there. I like being able to read it. I have no trouble reading the text (because it's a decent size and the layout handles zooming without issue.)
Also, the title of the article is repeated throughout and drastically emphasised (considering nothing else is that visually distinct and it even has a solid background color).
Using a line break between meta data and content is also a familiar paradigm. Spend 20 years reading your email with mutt or mailx and you'll catch on. ;)
The article's title is repeated throughout the comments list and emphasized. The other comment metadata has the same visual emphasis as the actual comments (except the link and author jump out more than the comment itself). The articles themselves expand to the full width of the window.