They didn't conclude that the licenses were not incompatible, they concluded that ZFS for Linux is not sufficiently based on the Linux kernel for the licenses to interact.
For example the GPL talks a lot about linking. Now how does that work with languages where there is no linking at build time (like Java)? The GPL's architects tells us that doesn't matter because its all in the same spirit. But that's just an opinion, you need a court to rule on that to be sure.