From the thread Apple have actively left the door open so other (currently non existent) OS could be booted, but as the author said, they’ve left it open but have not provided documentation.

From the thread:

“Linus himself has said he'd love to see Linux running on M1 macs, but doesn't think it'll happen. Not because they're locked down, but because Apple won't help us with documentation.

But they at least did this:

https://mobile.twitter.com/marcan42/status/13331260180689551...

The fact that Apple allows this in their secureboot policy is critical. Linux ports to game consoles / iPhones / etc are all fun and games, but you're always at the mercy of exploits, and the resulting cat and mouse game.

That means all effort is wasted unless exploits keep up, and regular users would never want to do this and severely limit their access to official upgrades and such. But this isn't an issue with M1 macs.”

Continues to discussions of limitations..