I am quite sure AMD is already using a fork of the Linux amdgpu KMD for their proprietary driver on Windows (which in part explains why a project like this can even be done)

Porting Linux drivers to anywhere is generally hard. It either makes your kernel very Linux-like (and lots of manpower to translate the abstractions), or forces you to put Linux on it. There are bazillions of projects to put recent Intel drivers on 9x and none suceeded, for example.

Plus my recent observation that for Haiku it was easier to port the out-of-tree nvidia driver and fork Mesa, than it was to port the Linux amdgpu one.