From the Hacker News thread
Incorrect. Apple's original presentations on Arm Macs suggested that booting other operating systems would be supported, and recently this has been confirmed by two of the engineers who worked on it. twitter.com/NikolajSchlej/status/133979… twitter.com/XenoKovah/status/1339914714… Although you cannot turn off secure boot on Arm Macs, Apple has deliberately provided a path by which you can prove that you really want to chainload to a user provided bootloader not signed by Apple. From there, obviously, it will be possible to load the kernel of another operating system, such as Linux.