They don't own the current code, but all contributions are based on diffs on the original code. The contributors own their changes, but not the code as a whole. I don't believe that it's ever been tested in court how many incremental changes are enough to remove the original copyright owners' copyright claims. Especially when it comes to something like ZFS where there are data structures that were designed a long time ago and they must be kept (as well as the code for handling them) for backwards compatibility essentially indefinitely.
The issue isn't removing the original copyright holders claim. No one is saying that's possible. Oracle is free to relicense the code they own at any time, but that relicense doesn't affect the code written by others and Oracle does not have rights to that code unless they had a CLA. The new OpenZFS code is not under a CLA.
