Converting Stadia version to proper desktop Linux version is a lot easier than converting DX11 or DX12 version to Vulkan itself. Bethesda developers gave a few talks about Stadia development and pointed that out.
Dealing with specific memory issues and etc. could be needed to address wider variety of desktop use cases. But it's not super hard to do for those who are already familiar with Vulkan development.
Converting DX12 to Vulkan is relatively simple, since the call structures and intent of the APIs are similar; DX11 to Vulkan takes more effort. Whether that is easier or not than providing high quality of optimisation across diverse hardware *will* depend on the application involved and the level of performance you are prepared to accept across diverse hardware. Bethesda games actually have a higher degree of complexity at the data manipulation layer ( the world model ) than many RPGs.
Regardless of that, it still requires non-trivial effort to go from Stadia to a *good*, and supportable desktop Linux implementation that will not be complained about, and you should not pretend otherwise.