Good question. I don't know, and am too lazy to look it up at the moment.
Even if there are outside contributors, if they are limited in number, they can be asked to sign off on a GPL-compatible license as well. Code contributions from hold-outs can be re-written in the worst-case.
Code contributions from hold-outs can be re-written
You mean reimplement an internal API? The legal feasibility of that depends on whether Oracle loses its appeal in another pending lawsuit.