I assume the idea is that, on average, you'd get better results just following those five points (focus on the body, get a reasonable text size, get decent line

I assume the idea is that, on average, you'd get better results just following those five points (focus on the body, get a reasonable text size, get decent line spacing, adjust the two for 45-90 characters per line, choose a decent font). When you spend reading the rest of the book, you'll realize that the author seems to deal with absolute bottom of the barrel typography regularly. Those five things alone would dramatically improve most of the bad examples the author brings up. A small improvement on a large scale is better than no improvement.