"Only". It would probably be easier for them to maintain the Linux drivers themselves than to thoroughly document every feature of the SoC.

It's not just about individual drivers though, it's about the surrounding kernel infrastructure and the whole desktop experience. For example, getting instant suspend/resume working on Linux is not (I'm fairly sure) just a matter of writing a driver for a particular bit of hardware.

They don't need to anything new. What they already have is enough. Their own software engineers could handle it fine. It's good enough for people who want this.

There are people who have clean room implemented entire nvidia drivers. Without doc. We can manage fine with whatever incomplete doc Apple allready has.

It's been a few years since I used Linux so forgive me if I'm off base here. But last time I used Linux with Nvidia, you had the choice of using FOSS drivers with mediocre performance, or having closed source drivers that performed on-par with Windows.