first of all, C++ can be exclusive to windows
second, nobody said this was exclusive to windows. people have said exactly the opposite
Its true, recently Microsoft is against closing off things, their pushing a large amount of windows exclusive stuff to android and iOS, one of those things is going to be the edge browser.
But Minecraft C++ will come to other platforms, but odds are the app is aimed for surface owners or people with windows tablets.
Microsoft will probably start another project of remaking the desktop Minecraft in C++ that has full support for servers and more than 7 people.
Odds are however that this C++ port will be renamed to something else and then the C++ Desktop version will also be a modern app but just be called Minecraft.