Both reimplement for compatibility at some level in the toolchain, why fair use analysis would privilege one level of interoperability over another is purely speculative; arguably, the kind of user-interoperability Oracle does is more of an assault on the market for what is reimplemented (that's the whole purpose of Oracle doing it) than what Google is doing, and that's a factor that weighs against fair use.

It is important to realize that since APIs have never been viewed as copyrightable previously, we also, if Google loses on both copyrightability and fair use, will have zero case law that is directly on point for any API reimplementation being fair use. Speculating on the law that will develop in that area is fun, but almost by definition not strongly grounded.