Eh, you may be right, but there's a big difference between "they are going to forbid other OSes by placing a software restriction where they explicitly permit things now" and "they already effectively forbid other OSes by not publishing developer documentation for proprietary hardware"--that's a tall order, and not a bar that many other hardware manufacturers meet either.

Like, could they lock down the bootloader? Sure. But that's effort they'd have to put in for minimal benefit at the moment. Opening up their hardware would be a lot more effort for questionable benefits (to Apple).