It's more of a hipster thing imo. For some people since it's minimalist and looks "old" , it must be good. Like I get keeping it simple but man it's CSS..
Some of it can be appealing, when basic ergonomic needs are met (readable text size and line length, adequate margins, and so forth). Most is just brutally pretentious, IMO.
Actually when I hit pages like this, I use the increase font size buttons. I tend to do this on phones too, especially. Yes, reader mode is also an option, but just bumping up the font size works too. You could also go back to the days when we had 800x600 monitors and 16px tended to be just the right size for that. ;-)
Who read article like this on mobile? In a pinch, I'd just activate Reader Mode (Safari, iOS), or more likely save it for reading on a bigger screen (tablet, laptop,…)
Read it on iOS Safari, without reader mode. Worked great.
Only thing that annoyed me is that there are very lengthy appendices. Thus the scroll bar suggests the main article is much longer than it actually is.
This page doesn’t specify _any_ font size. It relies on your browser to choose an appropriate size instead. If the text is too small for you to read, then your browser settings for default font size are wrong.
The conclusion I came to is that the audience is very tech-savvy and is used to activating Reader Mode when they encounter pages like this.