He makes a very valid point about trying to run regular windows alongside this beautiful interface. MS just couldn't resist the temptation to make it fully backwards compatible with their old software. I can almost guarantee it was some outside, higher C-level person (Balmer?) that "loved the new interface, but could we get it to run windows too?"
If MS knows what it's doing, it will make custom mobile versions of its office apps like Apple did and never try to shoehorn an interface designed for a mouse and keyboard into their mobile operating system again. But, alas, they lack the discipline to do it.