The Apple Platform Security[1] white paper describes the secure boot process for Apple silicon. The Mac boot process is significantly more configurable than the iOS boot process, and it allows operating in reduced security modes. (Including running locally signed operating systems.)
Apple knows how to build an iPhone: if they wanted to lock down a Mac they would have simply done that. There's something like nine pages detailing the differences. What word describes that other than "intentional" design? The fact that you can sign and boot a third party OS isn't an "accident" if it's documented, and there's no "exploit" because this is functionality the platform supports; anyone can do it with tools already present on the (Apple-signed) recovery OS.
They certainly don't provide great support for people wanting to develop [drivers for] these operating systems, but the platform was very clearly engineered to support booting them.
[1]: https://help.apple.com/pdf/security/en_US/apple-platform-sec...
I guess I'm missing something then. The Asahi blog says "Apple’s boot tooling will only work with what it considers to be a “valid” macOS installation inside an APFS container." Sounds very adversarial to "the ability to boot an arbitrary OS."
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.