I don’t want someone to tell me what paper I should use (size: Letter portrait). Frankly I don’t own letter-sized paper.
Also all sizes are in inches, which I don’t calculate in and wouldn’t know whether still sensible margins when printing on a different paper size.
That's pretty clearly stated near the top of the article. As a European, if I wrote a similar post I would do it all in metric and trust that my US readers were smart enough to substitute the units of their choice without whining about it. The concepts would not change, and a quick google would tell you that an inch is approx 25mm, which is enough for you to understand what's going on.
If you want to use these techniques on a global site, I think you could use multiple @page styles and switch between them with a dropdown like I did with portrait/landscape. Although A4 and letter are very close in size.