Phoronix published a review on the How Open-Source Allowed Valve To Implement VULKAN Much Faster On The Source 2 Engine A quote from the article:

While Linux users tend to prefer open-source hardware drivers out of philosophical beliefs or just making an easier out-of-the-box Linux experience, Valve and other early Vulkan stakeholders have yet another reason to appreciate open-source drivers as having an open-source driver allowed them to jump-start porting of the Source 2 Engine over to the new graphics API much faster and easier than if they were relying on a closed-source Vulkan driver. Here's the story that LunarG has exclusively shared with Phoronix about their process of bringing up Vulkan with open-source code.

Valve developers part of their Linux cabal have previously talked of how it's nice having the open-source Intel/Radeon Mesa/Gallium3D drivers when porting games/engines to Linux as they're able to analyze the OpenGL driver code when debugging an issue or trying to workaround a performance shortcoming. In the early days, Valve also allowed Intel's open-source driver developers access to the code-bases of some of their games so they could look at what the Source renderer is doing, etc, in order to optimize the open-source driver performance. Open-source wins and makes the development process easier on both sides of the table when not having to deal with binary blobs in the equation. With the bringing up of Vulkan over the past few months, open-source was the champion once again.

 How Open-Source Allowed Valve To Implement VULKAN Much Faster On The Source 2 Engine @ Phoronix