Processes--and a good example is business processes like SOPs, etc.--have inputs and outputs.
An API is a name for that process, plus a description of the required inputs.
Can you copyright a name and a description of inputs for a process?
The description and name matter a lot for this, and are typically quite general.
Now, the question is, where do we draw the line: As per Oracle, there cannot be any other character that does what Harry Potter does.
You specifically need to argue that the API itself - the specific combination of types in a specific order, with a given set of Unicode or ASCII characters to identify it - is not copyrightable, not just that it's made up of uncopyrightable things. This is harder, because this same practice in other contexts (e.g. music, literature, and so on) is very much protectable. You need to argue that software is different.