I'm an actual programmer and I consider API signatures copyrightable. I spend a significant amount of time as a lead developer laying out class structure and method names for a more junior developer to actually implement. APIs are clearly creative and not trivial work that should be protected.
I think copying an API in a compatibility layer should be protected as fair use, but I don't think you should be able to start your development by copying an API someone else created.
I think copying an API in a compatibility layer should be protected as fair use, but I don't think you should be able to start your development by copying an API someone else created.