The key thing to note about NVIDIA "open sourcing their driver" is that they only open sourced the kernel portion. I see no intention of opening the userspace portion. GPU drivers have multiple parts. The kernel driver is the low level interface that passes data to and from the hardware while the userspace is what actually handles converting OpenGL, Vulkan, OpenCL, CUDA, etc. calls into GPU commands and that part is where most of the performance impact happens. NVIDIA is not open sourcing the userspace.

That's why NVK/Nouveau are so important, because it is a fully open stack. It is also part of the Mesa project which encompasses all the open GPU drivers on Linux which makes it more integrated with the Linux graphics stack.