Does it mean we’ll not be able to purchase it through xbox live?
Windows 10 exclusivity is just for the PC version of the game. It just means you need to have the Windows10 operating system installed on your computer to download and play the game. This does not change anything about S3 on Xbox One.
It simply means that the PC version is only compatible with windows 10, since the Xbox app is a part of that operating system.
It means you can’t purchase it from your precious Steam service.