You have the wrong end of the stick here - Valve is dedicated to improving the Linux/*nix gaming scene, so they have hired driver developers, some from the existing Mesa open source community, though I wouldn't be surprised if they had branched out further.It's a fact. How else would Valve be able to make their ACO shader compiler be compatible with several AMD architectures ? Valve would be one of the last places I'd expect to see specialists in shader compilers ...
Valve are far from the company that released HL2:Episode 2 and Portal 2 so long ago - at this point it's barely permissable to call them a games developer.
They have become a platform provider, which means investing in hardware (Vive), and software (Steam OS, ACO, DXVK/Proton/Wine).
As to what ACO is compatible with, I suggest you dig a little deeper.
For now it only works with the RADV Vulkan driver, not AMDVLK Vulkan nor RadeonSI OpenGL - and it only works fully with Polaris and Vega (though V2 compatibilty is not stated to my knowledge).
Navi support is only partial at the moment, though obviously a priority now ACO is commited to Mesa master.
As to how Valve could do it, again they are using established driver devs, and AMD tend to document their GPU's uArch and ISA openly better than nVidia, though I could be wrong on the documentation part.