> Google LLC v Oracle America assumed (though didn't establish) that API's are copyrightable... BUT that developing against them falls under fair use, as long as the function implementations are independent.

That was only one prong of the four fair use considerations in that case. Look at Breyer's opinion, it does not say that copying APIs is fair use if implementations are independent, just that Google's specific usage in that instance met the four fair use considerations.

There are likely situations in which copying APIs is not fair use even if function implementations are independent, Breyer looked at substantiality of the code copied from Java, market effects and purpose and character of use.

If your goal is to copy APIs, and those APIs make up a substantial amount of code, and reimplement functions in order to skirt licenses and compete directly against the source work, or replace it, those three considerations might not be met and it might not be fair use. Breyer said Google copied a tiny fraction of code (<1%), its purpose was not to compete directly with Oracle but to build a mobile OS platform, and Google's reimplementation was not considered a replacement for Java.