Many researchers work on Windows, Swift has zero support there and when asked about it, they just kind of invite the Swift community to provide support.
So zero interest from either Apple or Google to make it work on Windows.
This is the latest status of Swift on Windows,
https://www.youtube.com/watch?v=Zjlxa1NIfJc
Then on Linux, Foundation support is still flaky, for basic stuff one still needs to do conditional compilation and directly import either Darwin or GLibc modules.
Swift is just like Objective-C, yes you can kind of use it outside Apple's ecosytem, but at expense of productivity and lack of adequate tooling.
Really I don't expect Tensorflow for Swift to ever reach mainstream adoption, maybe Google can sell it as Mac only and thus announce the project has having been a success.