A CLA and copyright assignment was how Oracle were able to make (now Oracle) ZFS proprietary again in the first place. As you say though, OpenZFS and Oracle ZFS have diverged quite a bit, and most of the world is now based around the OpenZFS variant that acts as the upstream for Linux, FreeBSD and even Windows variants.
They could relicense their codebase, of course, but the number of changes that have happened since they diverged is not small.