Earlier quoted context omitted.
When making a game for Stadia you can target a very small subset of hardware and test on those on one single linux distribution/software stack (at the moment all stadia servers are identical). When you make a game that anyone can download on their machine the amount of different hardware + os/driver combinations grows a lot and requires a lot more effort for a quality product.
The success of Stadia means more developers who are comfortable with Vulkan and linux as a platform. That should give us better ports and even more Vulkan first titles. That could lead to better driver support (at least from Nvidia :| ) on 'nix, and so on and so forth.
Sure the distance is smaller then starting with the Windows or Xbox/Playstation versions but there is still a lot of work to get it to work on any random pc someone builds.