It is generally different roles of people working on design guides like this versus working on documenting an API or other "basic functionality". It's not necessarily a zero sum effort (and probably often is entirely not) to produce design guides and work on other technical requirements at the same time.
Also, design, including and sometimes especially typography, is accessibility and is often worth that sort of attention to detail for exactly that reason, keeping designs accessible to all users. Defining contrasts and visual hierarchy hugely influences how accessible something using a design might be.
One specific example I see is the callout in this particular example for using much less UPPERCASE text moving forward from whatever their previous design language was. This is something I know that I've fought designers I've worked with over as an accessibility problem they've missed. (It's also something that Microsoft went through a huge accidental blow up in very public with early "Metro" designs that they had to apologize for and then walk back in consequent "Fluent" designs.) In general people read word shapes more than individual letters, uppercase forms a lot of very similar looking "rectangles" and even for readers with no other obvious dyslexia uppercase words are the visual equivalent of "speed bumps" in a roadway. They slow reading, sometimes to a crawl, and generally just get in the way, for all readers. (For dyslexic readers I've known they sometimes are even more "walls" than simply "speed bumps" and text entirely in uppercase will completely upset them, if not make the content 100% inaccessible to them and they will nope out entirely.)
That's an important thing about typography that is easy to miss. I've seen so many designs use all uppercase text in places simply because it "looks nice" (those "all words create about the same sort of rectangular shape" that makes a dyslexic viewpoint absolutely hate seeing all uppercase text add a regularity and "cleanliness" to design shapes that some designers find they love; one person's massive accessibility bug is another's aesthetic feature).