I had this question. Then I read the bit of the page with the sentence that starts ‘The quasi-proportional families:’

Possibly something similar to GitHub Monaspace's "texture healing" [1], i.e. variable-width characters individually centered on a fixed grid, for which e.g. an "m" is wider when placed next to an "i"?

...or maybe something much less sophisticated: it just keeps the appearance of the main fixed-width variant you'd use for code, but it's a slightly-variable width variant for plain text, where vertical alignment isn't important.

[1] https://github.com/githubnext/monaspace#texture-healing