Yes, if ZFS (CDDL) was integrated into Linux (GPL) then the GPL would need to apply to the CDDL files, which causes a conflict because the CDDL is not compatible with the GPL.

This isn't a problem integrating MIT code into a GPL project, because MIT's requirements are a subset of the GPL's requirements so the combined project being under the GPL is no problem. (Going the other way by integrating GPL code into an MIT project is technically also possible, but it would covert that project to a GPL project so most MIT projects would be resistant to this.)

This isn't a problem combining MIT and CDDL because both lack the GPL's virality. They can happily coexist in the same project, leaving each other alone.

(obligatory: I am not a lawyer)

And that's why zfs inches along with a fraction of the progress it could have had for decades.

This lack of required reciprocity and virtuous sounding "leave each other alone" is no virtue at all. It doesn't harm anyone else at least, which is great, but it's also shooting itself in the foot and a waste.