It could trivially be built for Linux if the author decided to (or released the code). But it's also more of a tech demo than a real utility; there's no indication it runs any existing GUI applications.
I honestly think it's more interesting as:
- A demonstration of what's possible within Unity (it's extremely impressive, because Unity's built-in GUI system is pretty terrible and it's clear that there's some level of composability, etc. to the UI system being demonstrated)
- An experiment in creating a totally virtual, cross-platform (gorgeous) desktop environment within a single app that could be trivially run on any OS
But of course it's still a long way from the latter, if someone even wanted to take it that direction