No, it's apparently compatible with Windows 7 or later. Remember, Office is targeted at business, and most businesses are still using Windows 7, and will be for a considerable time to come.
I believe that's going to change, drastically. Microsoft's path with Windows 10 (free updates from Windows => 7, as long as you do it within a year of release) is going to drive the fastest corporate OS migrations ever--for better, or for worse.
I know we're planning for it. It scares the hell out of us, but the incentive to move forward is so powerful there really isn't any other viable path.