Doesn't Skype have the same behaviour? Clicking "X" doesn't close the application.
Though I think here is a failure of Microsoft to improve the UI and use different symbols for closing the app and suspending it into the systray.
Either way, seems like a rather strict interpretation of 25.2. That said, I kind of welcome it. The principle of least surprise is something that should be much more strictly enforced and a lot of social apps make it way to easy to accidentally leak data into the public.