Earlier quoted context omitted.
> 80-char lines are too short for desktop displays Nothing makes them "too short for desktop displays". Text is not supposed to fill all/most of the available display width: “Anything from 45 to 75 characters is widely regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line(counting both letters and spaces) is widely regarded as ideal.” -- Th…
"The Elements of Typographic Style Applied to the Web" was originally released in 2005 and I'm afraid some of the advice may be stuck there. This advice being among that advice. In 2005 many displays were still 4:3 ratio and this advice more applicable. With modern 16:9 ratio screens 66 character line lengths can seem extremely short and require unnecessary vertical scrolling to view a message with such line length.…
Shorter lines are easier and faster to read and you don’t have to turn your head back and forth every line. This is what German Typographers would call a Bleiwüste, which means “lead desert” and translates to wall of text. And they call it that for a reason. Typographers use a wall of text when they want to disencourage users from reading it (legal text etc).
Column based layouts (like in newspapers) don’t work well with scolling pages, but limited line width certainly does. Scrolling doesn’t really cost the user much and on top of that smartphones limit the line width anyway, so users get a more consistent design.