It "is" a well-documented platform. I'm hoping that Apple makes it available to the open source community to make Linux on M1 happen sooner than later.
I assume you mean that Apple have it internally, but I wouldn't assume it's any good.
I would also love to see someone (I haven't got and can't afford one) try and fuzz it for undocumented instructions.
If you're talking about ptx, that's not the hardware ISA, but instead an IR for their shader compiler.
That doesn't operate on PTX but actual hardware binaries. Oddly, they ship disassemblers, but not assemblers.
And I think Apple knows macOS can't compete with Linux as a development environment against browser, Linux, and cloud-based deployments. Witness how they made a point of demonstrating a Linux VM running on Apple silicon during the M1 introductory keynote, and how they continue to remove dev-oriented tooling, allowing third-party setups like homebrew to fill it in.
It seems their goal is increasingly to focus on their end-user platform only, which for dev tooling means only focusing on XCode/etc. and not the Mac's capabilities for other deployment targets.
... which leaves an obvious gap for Linux to fill. In fact, given how large and capable Apple is these days, I think there's a good chance they'll put a bit more effort into helping Linux integration along (especially Linux in VMs on macOS -- that's probably the main plan at the moment). They realize they only stand to gain from such efforts.