It basically just has to look like macOS in some trivial sense, it doesn't have to be macOS, there are no obstacles. The system is designed specifically to enable booting custom compiled kernels and former members of the Apple team have said booting other OSes was intentionally left open. The company just doesn't make any guarantees about that.
Where is it stated in official Apple communications that Apple laptops will always support other OSes like Linux?
There you have it.
If they don't guarantee it, then better not depend on it. Or waste your time on it.
This also holds for members of the Asahi team. Unless they don't mind that a decade or so of their working life goes down the drain in one decision from Apple management or Apple lawyers.