Open source licenses are enforced by copyright; without owning the copyright of a work there's not much you are able to license with respect to copying. There's a reason most license files out there start with "(c) 2019 <Authors>".
The opposite of a copyrighted work is not an open sourced work: it's a work in the public domain.
> So that would mean that Android would have to be under GPL, right?
That kind of depends on whether you can apply a different license to an API and its implementation. There's not really any precedent for that kind of question, because it presupposes the validity of a kind of copyright that the industry has grown up assuming is not valid.
AFAIK Android originally didn't use OpenJDK, but rather Apache Harmony, which is an IBM reimplementation of the Java JDK. So in that sense maybe Oracle does have a point (if API's are deemed to be copyrightable).
I think Android has since switched to OpenJDK, so it seems to me what Oracle and Google are arguing about is whether Google owes Oracle some damages for their previous use of Harmony. And of course, the lawsuit could have larger repercussions as well, if API's are deemed to be copyrightable.
Now I am likewise confused: how can it be copyrighted if it's been open sourced?