An API is the knowledge of what methods are available, how to call them and what they do. How is that not fact?

While I recognise that creating a good API is indeed a difficult task

Unless you want to argue that everything ever created becomes a fact, in which case we can just ditch copyright law, and software licenses right away.

Definitions, phone numbers, addresses and algorithms are all invented, yet are all facts or ideas, and none can be Copyrighted.