That's mostly true but not absolutely so. Windows kernel space needs signed-code in 64-bits (except in a configuration only suitable for testing during dev), an

That's mostly true but not absolutely so. Windows kernel space needs signed-code in 64-bits (except in a configuration only suitable for testing during dev), and MS very much makes use of internal undocumented and changing API to implement various high level features in a way that gives them a virtual exclusivity by the mere fact they are working at a level such that no competitor can even appear, or if a competitor do appear it will have the greatest pain to make a correct use of undocumented API and the competing implementation might not even continue to work after applying even security patches. Look for example at WSL: while it is very very cool and useful, no third-party can (at least in practice without MS collaboration) write a competing implementation. The same happened throughout the history of the dev of windows in various degrees and fields, and MS even has been found guilty of violating various anti-trust laws despite arguing in court that bundling disputed functionalities was a necessity of the design of their OS (they sometimes eventually managed to release extra stripped ISO extremely minimally modified compared to original ones to comply, making it clear that they are technically way better than what they argue in court...) So it's a mere question of size and market share -- and it is not possible to reason from pure logic and as if it happened in a vacuum: the reason why they should not have an absolute unconditional right to design their exclusive features by implementing them using internal-only API is simply because they are a gigantic corporation shipping a behemoth OS used on a ton of computers and that would allow them to in practice and because of the clear advantage of maintaining the two sides of APIs extinguish any competition in the field they want on their platform.