> First, it seems to likely show that the advice that it's very important to keep line lengths short is incorrect since, if that had a very large impact, it would've overwhelmed the other changes and resulted in reduced engagement and not increased engagement.
Well, I can't speak for anyone else, but I read all articles on that site with Reader view since they're impossible to read any other way. There's good content there, but man is it a pain in the ass to read!
I'm feeling like you did exactly what you should have done. You chose to read the site in a style (Reader Mode) that benefits you. Maybe someone else chooses a style, set of fonts, or overrides the margins manually.
I think we've gone too far with HTML styling. I'd much prefer unstyled semantic markup and then let my theme choices display the content. Early HTML was quite a bit more like this. Gopher protocol and Project Gemini might be other examples of content without "design."
That’s probably zero.
Why would you even make people have to take an extra step to comfortably read your content?