Earlier quoted context omitted.
Judging from the author's other assertions, this suggestion probably comes from a place of not limiting yourself to system fonts given that they're so prevalent. I can't say I agree either, and would add that some of the "free fonts that inundate the internet" are professionally designed by well-respected foundries. That said, perhaps we should view these "rules" as laying a foundation for people who know nothing. I…
Yeah IIRC he has a page elsewhere listing system fonts and proposing alternatives, including several free fonts. I'm pretty sure the reason is just that free fonts tend not to be "full featured." His typographic style is based on classic printing, so it really depends on the font having features like small caps, tabular, lining, & proportional number options, etc.
Re: Typography in Ten Minutes
#11This is the list: https://practicaltypography.com/font-recommendations.html