He is using a font size that's a bit too big. And there's too much spacing, you can configure all aspects.
I get about 8-10 characters on these devices, the font isn't too small and the spacing is fine. With my settings I get about 70% of a book's actual page (I use the same book to compare) which is a fair compromise imo.
It's fast to read shorter lines anyway because there's less horizontal travel distance. I actually think this aspect ratio is pretty great for reading. I also have a Kobo Clara 2E which has been relegated to just reading by the pool/bath/beach.
Also, these devices are pretty good for manga reading if you're into that.
Sorry, maybe I'm dumb, but what does that mean? 8-10 characters per...?
The lack of these is most noticeable when text is in narrower columns. But even with larger e-readers it is, IMO, responsible for a lot of the “this still feels less readable than a book for some reason” vibe that ebooks (and lots of self-published paper books) still have.
I really lament that in the switch to electronic text we seem to have forgotten what were thought of as basic requirements for readability in paper-based publication.