When I joined MS my boss told me that Microsoft apps weren't allowed to use internal APIs, I think as part of the anti-trust settlement. Windows components (e.g. the Settings app) can, but actual apps (e.g. Word) couldn't do anything third-party developers wouldn't be allowed to do. Some APIs are considered public but their documentation is less than useful, admittedly. But with those I don't have access to anything better either. :/
(Disclaimer: I speak for myself, based on my memory of public knowledge.)