Compatible until MS breaks it
Lowering the friction required to move apps from Windows is one way to ease the process of selling Linux to managements as a viable server host.
Microsoft know this. I suspect this 3rd party has been a lot more helpful to Mono's developers than MS have, beyond selling them a set of manuals.
MS have know for decades that controlling their interfaces (and choosing when and how they change them) is a key tool to stopping anyone moving. A frozen interface is a copyable interface and hence offers potential for migration.
If they do see re-hosting (IE windows licensing from big users drops as they re-host) expect the usual MS responses. MS can innovate (well they can buy someone who can innovate) when they want to. Expect a new .Net spec with some new features (probably ones developers have been clamouring for for years) and some pointless stuff which will waste developer time all with various parameter orders shuffled but which will break Mono quite effectively for some time.
Mine will be the one with various inside Microsoft books in the pockets.