Or, not every UI library is intended for use cases where a13y even makes sense.

Like a debug UI in a game engine, or in an embedded device that doesn't even have input for a13y.

Being accessible to the intended users always matters. If you think it doesn't, that probably means it's currently accessible to those users (or that those who are it is inaccessible to have filtered themselves out, and are no longer users).

For example - in your debug UI, colorblind-friendly colors don't matter, until you hire your twelfth member of the team, who struggles to tell red and green apart.

This library's default is greyscale anyway, so it's by default colorblind friendly.