Okay, the answer was pretty clear: as long as we have Krita in any stores, we cannot use Mixbox. They offered two suggestions, one was we create a closed-source plugin for Krita that would only be available in the stores, or they would fork part of Krita and create a plugin that links to mixbox and distribute that themselves. Both would violate the GPLv3.
So, yes, GitHub - joeedh/pigment-painter is probably the only way forward for anyone who wants to implement this in Krita.