Using the edges of the pages for navigation as the only option would be ridiculous, but it's not; there are conventional navigation links. Think about reading this on a tablet, where a lot of ebook readers have a convention of turning "pages" by touching the left or right side of the screen, and this may make more sense.
Underlining in the typewriter style is a sin against typography. :) I don't like the way Butterick's chosen to do links here, though; the lozenge mark seems kind of twee, like he's choosing to make a stand against both underlining and color changes at the expense of both readability and learned behavior. (Most of us by now associate both underlines and blue inline text, whether in conjunction or separately, with "click this to make something happen.")
I accidentally navigated away from the page I was reading about 8 times (clicking back each time) through stray clicks on the left or right margin where there wasn't anything there.
Underlining in the typewriter style is a sin against typography. :) I don't like the way Butterick's chosen to do links here, though; the lozenge mark seems kind of twee, like he's choosing to make a stand against both underlining and color changes at the expense of both readability and learned behavior. (Most of us by now associate both underlines and blue inline text, whether in conjunction or separately, with "click this to make something happen.")