The driver fixes are usually performance improvements where the vendor rewrites shaders to make the game run better or add support for weird vendor features (like SLI). If the vendor had to hack around the game to make it work at all, the developers wouldn't have been able to even test the game during development.

WebRender was locked to NVIDIA as a known target with a known set of driver bugs. They could've chosen AMD or Intel as the target instead and worked to figure out all the relevant driver bugs and worked around them, but for whatever reason they picked NV.