I would never buy a Mac, but what's the issue with supporting Mx processors?

Are they a generic ARM platform or something highly proprietary with ISA extensions and the like?

And if Apple is pulling a Nintendo here why is this project allowed to exist in the first place? It's not like they are getting hit with an anti-trust any time soon.

The problem are really not the CPU cores itself. It's a generic arm core in terms of ISA with just a tiny bit of proprietary extensions. The problem are all the peripherals. GPU, NPU, Display, USB, Wifi, HID, sound etc etc. These all require custom drivers and reverse engineering.