Yes, interoperability is relevant, because it affects the necessity of copying that particular code rather than making one's own substitute, which in turn affects fair use.
But Google's lawyers (inexplicably, in my opinion) failed to talk much about the fact that many Java libraries are interoperable between Android and Oracle Java, leaving the courts to think only in terms of full applications which are not interoperable. Thus the courts have treated this case as if the only benefit to Android's reuse of Java was programmer familiarity.
Thankfully, Google won anyway, so any defendant in a future case who can make a better interoperability argument will be in an even stronger position.
Edit: For example, in the Federal Circuit ruling that the Supreme Court just overruled, they complain: "Indeed, given the record evidence that Google designed Android so that it would not be compatible with the Java platform, or the JVM specifically, we find Google's interoperability argument confusing. [..Google] points to no Java apps that either pre-dated or post-dated Android that could run on the Android platform." [1]
Google didn't argue on interoperability since that would have torpedoed their core arguments that Android is a transformative work creating a new market outside of, and different from, Sun Java Standard Edition.
To argue interoperability Google would have needed to copy the entire JAVA SE API.
The key difference is that Java SE (designed for desktops) API was considered by Google mostly not required on smartphone/mobile devices envisaged for Android. Sun would only licence Java SE complete (Sun was the one wanting complete interoperability).
To the extent the concept of interoperability enters into it, it was on the human side; the arguments were about leveraging existing programmer knowledge to the extent that Android's requirements were shared with and common to Java SE.
> To argue interoperability Google would have needed to copy the entire JAVA SE API.
Only if Google wanted to argue interoperability as defined by Sun/Oracle. I've never seen a coherent argument why Sun's TCK should be considered the sole authority on what degree of interoperability should have legal significance in this copyright case, particularly given that Sun's TCK was part of their trademark licensing program.
And there are obvious reasons why a court would shy away from letting something like Sun's TCK be used as part of a significant legal test; for example, it's really awkward for legal purposes to define something as copyright infringement while it's a work in progress, but it suddenly becomes okay as soon as it attains the status of being 100% compatible and bug-free. It's also not clear how the law could reasonably handle a definition of interoperability that Sun/Oracle can unilaterally make into a moving target and add arbitrary requirements to.
But Google's lawyers (inexplicably, in my opinion) failed to talk much about the fact that many Java libraries are interoperable between Android and Oracle Java, leaving the courts to think only in terms of full applications which are not interoperable. Thus the courts have treated this case as if the only benefit to Android's reuse of Java was programmer familiarity.
Thankfully, Google won anyway, so any defendant in a future case who can make a better interoperability argument will be in an even stronger position.
Edit: For example, in the Federal Circuit ruling that the Supreme Court just overruled, they complain: "Indeed, given the record evidence that Google designed Android so that it would not be compatible with the Java platform, or the JVM specifically, we find Google's interoperability argument confusing. [..Google] points to no Java apps that either pre-dated or post-dated Android that could run on the Android platform." [1]
[1] http://www.cafc.uscourts.gov/sites/default/files/opinions-or...