They didn't actually say whether APIs are copyrightable, just that they would assume it for sake of argument and test to see if the fair use test was sufficient. They found the fair use test sufficient so they didn't investigate whether it was copyrightable or not.
> In 2014, a federal appeals court ruled that the APIs could be subject to copyright in a controversial decision overturning a ruling by Judge William Alsup. (The Supreme Court declined to hear Google’s appeal the following year, letting the appellate ruling stand.)
