Agreed, most of the code can likely, with some effort, remain platform-agnostic. These guys were able to port to XBox, PS and Switch after all.
I assume Google Stadia's multiprocessor structure is very similar to that of an ordinary pc, I know that experienced engine programmers can get that little extra performance for XBox and PS if they can map asses memory according to actual hardware layout.
Stadia might also provide an alternative for windowing system (e.g. X on Linux) so that frames are directly sent over ip, reducing the application to a console program, though that seems a bit unlikely. Even then that's only a small part of the entire engine.
Again, I haven't seen the API.
What interests me is this: will BG3 support Google Stadia controller? Cause that would surely imply a release for Xbox/PS later on.
Would be a shame to get the controls right and then not reuse it.