Ubuntu Linux support has been there from the beginning, but we did not fully integrate it into the App Builder early on. Since 2015 our nightly builds also run Linux platform conversion and check if everything works. So it works internally for us right now. Work continues here after the release of our first own product, which will also be released on Linux on Steam.
Since all source code for PC platforms is available (early 2014 release) and free to use, you can also try compiling directly on Linux (best to use OpenTK and fix any remaining linking issues with it) and your game should work just fine :) More advanced Linux features are coming in the future.