I'm a bit shocked that it would even be possible for a third party to implement such a thing without rebuilding Windows itself. Is it a matter of implementing "device drivers"?
My understanding is that drivers aren't part of the kernel in Windows like they are in Linux. And I believe an ARM build of some kind or another already exists out there for Windows
Windows on ARM isn't new. The oldest Windows on ARM was Windows 8 RT (ARM32 only, I believe). Right now it seems Windows 10 and Windows 11 have ARM64 versions, but there was also Windows IoT Core from the Windows IoT OS branch a few years ago.