Just shrink your window if you want 80 chars per line, jeez.
So to me the advice makes things worse, not better. 60-80 chars per line???? Even my email is wider than that. Go for at least 200.
This stuff with narrow lines may have been tested on poor readers perhaps? I don't know. All I know is I hate narrow lines - and I especially hate them on websites.
Also, none of those 7 even did a proper A/B/A test on what they claim --while the counter-example is well shown in tons of tests.