I'm not sure about that. API design does require a significant creative effort, with a set of skills almost disjoint from those involved in implementation, so one could reasonably say it's a "form of expression" in its own right.

However, the whole purpose of API design is to allow people on one side to use it without worrying about implementation, and people on the other side to implement it without worrying about who's going to use it, so I would argue that anybody doing either of those things has a reasonable claim of "fair use".

What would not be "fair use" is copying it and representing it as your own work, or copying it with significant unauthorized changes and attributing the modified version to the original author.