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]

[1] http://www.cafc.uscourts.gov/sites/default/files/opinions-or...