Honestly this stands contrary to almost every principle of fair use (transformative work of different character for the purpose of comment, parody etc.).

Google took tons of APIs from a platform and implemented them into... a platform.

If you think designing thousands of classes is not substantial that's a very different argument, different from fair use.

Fair use means yes, APIs are copyrightable, but this is transformative use. And, to anyone with a clue in software dev... no it's not.

It's basically like taking someone else's script as-is and shooting a movie from it, and the court deeming this use of a script "fair use".