Microsoft has the culture and the technology to tell private and public APIs apart and to check code across the company to ensure that only public APIs are called. This was required for decades as part of the Department of Justice consent decree and every single product in the company had scanners to check that they weren't using any private APIs (or similar hacks to get access to them such as privately searching for symbols in Windows DLL files). This was drilled into the heads of everyone, including what I assume are 90% of VP+ people currently at the company, for a very long time.
For them to do this is a conscious decision to be anticompetitive.