Also, you are distorting the argument by making it about one method signature instead of thousands of interconnected types and methods that work together to present an ergonomic interface for the developer.
Two equally good programmers may come up with radically different interfaces. Whereas if given an interface and instructed to implement it, they are likely to converge to a similar implementation.
A table of baseball batting averages or telephone numbers is not copyrightable, no matter how good or bad it is organized.
There's a reason the Supreme Court didn't adopt the argument you are pushing and instead ruled on fair use grounds: It's a bad argument because a large API is clearly a creative work.