How does this have anything to do with the new Microsoft? Unreal has always been a windows based engine. Of course it currently runs on many more platforms but Windows/Visual Studio is where its roots are.

I think you're misinterpreting this, UE supported VS 2013, this is simply to announce support for the newer version of VS[1].

VS 2015 was only released a few months ago, it usually takes a few months before everyone even starts to moves over.

[1] https://forums.unrealengine.com/showthread.php?77332-UE4-and...