I'm not how this is related to not having any knobs to tune. Aren't there many ways for code to be consistent with pep8? Isn't advocating for only one version of pep8-consistent code in essence an attempt to supercede pep8?
If there are no knobs to turn (like in gofmt) everyone just has the same settings and you don't have to make sure everyone sets the knobs to the same values.
In this case, if black becomes the standard for defining appropriate style then it is superceding pep8 as the standard for style (many styles consistent with pep8 are not black outputs, so if black's style becomes mandatory, it is a ruling against these previously accepted alternatives).