Well, if Apple builds an app in C# then they would probably want to use their OS to do so. In order to do that they would need Visual Studio for Mac which would

Well, if Apple builds an app in C# then they would probably want to use their OS to do so. In order to do that they would need Visual Studio for Mac which would allow them to work with Apple to improve those tools and open doors for developing C# applications on MacOS as well as compiling those Applications for MacOS. If Apple were to write a client in Swift or ObjC then it would do the reverse and potentially open up applications to be compiled for and run on Windows. This allowing easier porting of apps from MacOS to Windows and helping eliminate their "lack of apps in the Windows store" problem. Cross functionality is Microsoft's current game. How many different ways they can achieve it is up to your imagination.