It's still their project and the Oracle Contributor Agreement means they get to asset joint ownership of your contributions.

That's broadly the point of CLAs, but for a beefy project like OpenJDK with so much shared code baked deep into enterprise deployment, Oracle will feel it's critical they can pull freely given code into the depths of their closed Java builds.

It's their project. It does absolutely block contributions (employers are unhappy sacrificing their engineering output to Oracle). If you don't like it, fork it.