If they happen to be lucky enough to use the cherry picked features from OpenJDK (cause they are now using it, but only the parts/packages they care about), not taking advantage of newer JVM bytecodes and ideally written in a Java 7 style.

I don’t disagree at all, but the question isn’t “are there pieces of existing software that run on both” but “is that the reason that Google copied the API or is it a side effect?”

I'd argue that that is the wrong question, motive doesn't come into play in this law (specifically: The merger doctrine). You should rather be asking the hypothetical version. If someone tried to make a second system that could run the software, would they have had to copy the API? If so the API is not copyrightable.

The existence of those works that run on both means you really need to start coughing up evidence for your repeated assertion that Google's motives were strictly about something else.