IIRC Marcan mentioned something he found that had been deliberately put into the Mac boot loader that made booting alternative operating systems easier and perhaps making it possible altogether.

That's apparent enough from the fact that you don't need a jailbreak exploit to boot non-Apple-signed kernels on a Mac, unlike iPads with exactly the same silicon. They are intentionally configured differently.

xeno-kovah is responsible for that one. see the most excellent 39c3 video "asahi linux porting linux to apple silicon" for more information.