An API design is also a creative and highly stylistic work. Anyone who has ever designed a large API, or suffered a poorly designed one, can attest to that. A bad design can work just as well as a good design.
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.
Arguably API design is the most creative aspect of programming. The space of possible interfaces is infinite, whereas function implementations are heavily constrained by the interface and often there is only a handful of ways to write an implementation for a given interface.
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.
An API design is plenty creative. The bar for what is creative enough to get copyright protection is extremely low, basically zero. An alphabetized phone book is literally the example people have to use for something that isn't.
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.
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.