Apple did not lock the bootloader, but they do not provide documentation for their products.

What would have been a trivial porting work with documentation, becomes extremely time-consuming and hard work without documentation.

That is why Asahi Linux lags by several years with the support for Apple computers, and it is unlikely that this lag time will ever be reduced. Even for the old Apple computers the hardware support is only partial, so such computers are never as useful for running Linux as AMD/Intel based computers.