This is an amazing suggestion. Whether the UI is actually great or not, the fact is that Microsoft has a large, world-class usability research team, some of the best testing labs anywhere, and does basic research on cutting edge UI. Now, are they designers? There are designers, for sure, but they also farm out lots of design to many of the best design studios in San Francisco and New York.
So the question is not whether Microsoft THINKS about UI, the question is whether that thinking results in effective and delightful UI.