Well of course, there’s probably other Windows stuff they are also using, considering how much they aren’t even trying to ship for something like Ubuntu, which would be super easy otherwise.

I can only imagine how big of a push Autodesk can easily put towards Linux. That would easily make the current rise to 5% be nothing in comparison. Maybe MS is paying them too, to keep them together.
Of course it might also just be that MS makes it easier for them to setup a DRM (Digital Rights Management) as compared to Linux, not that it matters considering how much they have been pirated.