Internal API is not an SDK. It regularly astounds me how many commenters on a so-called hacker forum seem to think it is. Is everyone else publishing their tightly coupled business logic code as API these days? Is that what GraphQL is?
This isn't exposing business logic, this is an operating system vendor deciding what it exposes to vendors. There is clearly an API that the apple watch is using to communicate with the OS, why shouldn't other vendors be allowed to access this?
If you're astounded by hackers asking practical questions, maybe you should stop carrying water for corps and see how your back feels. Let's talk shop, what are the roadblocks Apple faces here?