This is not a much a question of the API as it is of the effect during execution. If the API would be considered to be what was protected then copyleft code can never be cloned in an interoperable manner under permissive licenses.
Instead the idea is that the code linking to GPL'ed code generates a new work during execution that's a derative of it AND the GPL'ed work, inheriting the copyleft license, requiring a compatible license on the linking code.
Instead the idea is that the code linking to GPL'ed code generates a new work during execution that's a derative of it AND the GPL'ed work, inheriting the copyleft license, requiring a compatible license on the linking code.