No documentation is being hostile to other OSes running on their hardware.
As a long time mac user, the idea of them supporting other operating systems seems really un-Appley to me. What happens if I put linux on my (still under warranty) macbook and then have to take it in to an Apple store for some reason?
I would guess that if you showed up with a mac under warranty but with a custom OS installed, they would boot macos in some fashion and run whatever their diagnostic tool is, and if the hardware checks out they'll tell you to buzz off.
I guess my point overall is that Apple seems to be pretty intent on curating and supporting a specific user experience with their products (which is a big part of their pre-occupation with controlling everything top to bottom in the first place), and supporting 3rd party OS's seemed at odds with that.
