The problem is that AMD treats these as custom products, even though they don't have to. This was something Nvidia was always much better at, since their normal Windows drivers always worked with the Mac GPUs. And while it might be custom hardware, its still using the same driver as any other GPU. AMD simply blacklists the regular drivers from being installed on Apple hardware. Which can be trivially circumvented by editing the driver's installer configuration.
Personally, I am quite disappointed that AMD chooses to treat products they make for Apple differently from the other Windows products. All probably because of some paranoid lawyer in the company. They could at least offer these as beta drivers with no support...