The reason why SCOTUS bent over backwards to not talk about copyrightability was
not because they assumed it was true for APIs, but because they didn't feel like they had all the facts. They basically said "we don't know if it's copyrightable, but if it is, here's a ruling that makes this case and anything similar to it go away".
Oracle only has copyright over APIs in the Federal Circuit, because they were able to hoodwink the judge into applying patent logic[0] to a copyright case. In other circuits it's still up in the air. And in the Ninth Circuit[1] there's already loads of controlling precedent that would have resulted in Oracle's case being summarily dismissed, API copyright or no.
The term "thin copyright" is a term of art. It refers to the kind of copyright protection you get from combining uncopyrightable elements in a creative way. For example, you can't own a particular chord progression. But, if you combine that with, say, a particular instrument, some audio engineering techniques, the subject matter of the lyrics, and so on... then you start getting something that requires creative effort and thus is copyrightable. Courts still have to take this into account when ruling on copyright claims as they do not want to give people a monopoly over just the chord, or just that instrument, etc.
In the case of APIs, we're talking about a series of names, plus an arrangement of type signatures that go with them. Very much a thin copyright, as the legal profession in the US calls it.
And when you have thin copyright, courts are going to be more liberal with handing out fair use exceptions. The "programmer convenience" argument that SCOTUS adopted means that copying an API to put in a different platform is OK. The Ninth Circuit says that copying an API to reimplement a platform that other people's code relies upon is also OK. There's very little room left to actually make a copyright claim on an API alone.
In the case of Copilot, it's not merely copying APIs and filling them out with novel details. It is either generating wholly novel code, or regurgitating training data, the latter of which is just a regular 'ol infringement claim with no difficult legal questions to worry about.
[0] The Court of Appeals for the Federal Circuit is the only court with subject-matter jurisdiction over patent claims. When you're the only person who can make hammers, everything looks like a nail.
[1] The Ninth Circuit court of appeals has jurisdiction over California, which means it takes on the brunt of copyright cases.