Valve is contributing quite a bit, with their ACO compiler for instance, but the RADV driver did not in fact originate with them. We owe a great deal to David Airlie for that. He wrote a complete vulkan driver years before AMD released the source code for AMDVLK. Valve started contributing a few years in, IIRC.
To AMD's credit, they did help with documentation, questions, and much of the work was built on top of what was there for RadeonSI (OpenGL): shader compiler back-end, etc.
