True, but they had to implement their own bootloader chain and because of such overhead they need a lot of effort to port to each new apple SoC generation

That is the reality for huge amount of ARM powered hardware, unless you fancy running vendor forks of kernel, u-boot, etc.

Ok.. and? That's job someone has already done, so what does it matter?

From what I've understood there's significant backwards compatibility for the new SoCs, so the significant work they need to do is to support new features, not getting things running.