Re: no such rule previously existed
Who the publisher is doesn't matter. Was the API published? Was the API published with the intent that it be **used** by others?
As the Supreme Court made clear over one hundred and forty years ago in Baker v. Selden:
"The description of the art in a book, though entitled to the benefit of copyright, lays no foundation for an exclusive claim to the art itself. The object of the one is explanation; the object of the other is use. The former may be secured by copyright. The latter can only be secured, if it can be secured at all, by letters-patent."
A book explaining the API may be protected by copyright. The actual API it describes - the useful art - can only be protected by patents. Oracle tried **and failed** to get a patent on the Java API.
Your argument that an API isn't the same as a word in a dictionary is just...huh? What? It's word salad.
Context is irrelevant. Definitions are irrelevant. Leaving aside that you cannot copyright a language - and Java is a language - the function names in an API are a method of operation: the means of controlling the behaviour and outputs of a (virtual) machine. They cannot be protected by copyright, per 102(b):
"(b)In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work."
The API is part of the software in the same way that the brake pedal is part of a car - you don't get to insist that everyone else's brake pedal has to look different from yours and go in the boot just because you were the first to put a lever by the driver's feet. Get a patent or go home.
You need to spend more time understanding the law, programming and well - everything.
Sorry.