What does making a different choice for GUI and CLI apps achieve, other than making it impossible to ever have a single coherent place to look/synchronise/etc?

What does making a different choice for GUI and CLI apps achieve, other than making it impossible to ever have a single coherent place to look/synchronise/etc? XDG's spec doesn't make such a distinction, so you’re advocating for a new competing standard. I don't think Apple’s documentation intends to cover command line programs, but… in the absence of any concrete rule, putting your files where most programs on the system do is a reasonable choice, and that place will never be ~/.config/ on macOS. Apps like homebrew that default to the library but support XDG_CONFIG_HOME if explicitly set, are a decent compromise, though.