That’s true but that’s probably only so that it wouldn’t have been a subject when Apple Silicon Macs were released because Intel Macs weren’t locked.
In reality, the bootloader isn’t closed (yet) but the hardware is so much undocumented that it’s easy to understand that Apple doesn’t want anything else than their OS on your mac. The « alternative os » situation is actually worse than it used to be with Intel Macs and Apple is paying a lot of attention in never talking about this "feature".
IMO, they will just quietly remove this possibility on new generations when everyone will have forgotten that boot camp used to be a thing.