They could rewrite all the code, and then change the license. Patents might still apply (but patents are short enough that I expect if any existed they have expired). However ZFS is a lot of code that is often tricky to get right. It will be really hard to rewrite in a way that the courts don't (reasonably/correctly) say wasn't a rewrite it was just moving some lines so you can claim ownership, but it is possible. By the time anyone knows enough about zfs that they could attempt this they are also too tainted by the existing code.
So of course they won't, but it isn't impossible.