I’m going to play devil’s advocate.
It’s not that Microsoft doesn’t get tablets, or that it’s “in denial” about them, Microsoft simply categorizes them differently. Microsoft views tablets as PCs because it takes the term “PC” literally. In Microsoft’s mind, tablets are PCs because “PC” stands for Personal Computer. And when it comes down to it, Microsoft is right. Tablets are Personal Computers. They’re actually the most personal Personal Computers we’ve ever seen.
Apple, on the other hand, categorizes tablets differently. It argues that PCs (which include Macs) are the things that sit on our desks and that any other type of computer is a Post-PC device. Tablets, by that measure, are Post-PC devices because they offer a totally different computing experience than traditional PCs do.
In the end, Microsoft and Apple are both right. The only reason we call tablets Post-PC devices rather than PCs is because Apple got to us first.