IIRC, The GPL incompatibility prohibits distribution of linked binaries. Since the CDDL-licensed ZFS kernel modules have to be linked against the kernel, those compiled modules can't be distributed.

Hence, Canonical (or anyone else trying to re-release under the CDDL) cannot distribute binaries of the kernel modules.

However, you are fully allowed to compile and link the two on your own. This is why the alternative to binary distribution is compiling yourself with the help of DKMS.