They don't cover just visual design, but design in general. While visual interfaces are most commonly associated with design, design is actually related to many other areas as well.
Latin is related to many things, but surely we can agree it's not a "must learn" for someone designing a device driver.
Something as general as "don't make me think" may make sense for API design, but honestly I doubt the applicability of "The Elements of Typographic Style" or "Grid Systems" to library documentation and API discoverability.