One assumes that if Oracle decided to do its ZFS on Linux, it would do so under an actual compatible license. Since Oracle owns Oracle's ZFS, they can just release it under GPLv2 (or MIT, BSD, whatever) and not be even potentially doing anything that would let someone sue them.
There is no "just" in "just release it under [another license]". I'd written a detailed rebuttal of why it's not that simple in the GP post to yours: https://news.ycombinator.com/item?id=24331156
I did forget the amount of effort to port OracleZFS to work with Linux, true, but I stand by the claim that they could release their own ZFS code under any license they want. They wouldn't have to "contact all 3rd party contributors and get the to agree to a license change", because Sun only took code that was legally signed over to them. The resulting OracleZFS would be missing all OpenZFS code (because indeed that code would only be available under the CDDL), but insofar as that it was missing code that Oracle didn't own it could be licensed at will. This would leave them having to invest the effort to port OracleZFS to run in Linux, which is a valid argument against them taking this approach.