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.

That is irrelevant for the laptops Asahi currently support or are working on supporting.

It’s the difference between “there’s no published standards other than the reference implementation for some of the API” and “there’s no published standards and extremely monolithic reference implementation design coupled to proprietary blobs and considerable effort spent on signing/boot chain authentication to prevent third party implementations”. Apple is currently the former.