Could you explain? FreeBSD was not the reference implementation of ZFS, illumos was. ZoL is still tied up in the OpenZFS project (as are illumos and FreeBSD), this is more a matter of rebasing onto the subproject where more development is occurring. As it was, FreeBSD's implementation was already using a Solaris/illumos compatibility shim, and it sounds like switching isn't that much work.
What does the solaris compatibility shim do in the end, which couldn't be done direct in the codebase? I don't like shims, it feels like the linux compatibility ABI moment where something gets added and the shim drifts, and then you can't do eg DRI any more because "they changed how it worked and we didn't catch up"
But, thanks for the cluestick hit. If this is just flavour of ice-cream from the same truck, I'm less worried.
On a separate note, illumos is a really cool project. I hope this transition (and apparent reduced activity in their development on ZFS) is not a symptom of the project losing momentum. My first experiences with a computer were on SunOS and early Solaris, and illumos is the the [open source] heir to that throne.
The heartening moment, is that linux and BSD should share code when they can, because its sensiblet to avoid pointless fork. Pointful fork, (and I exclude licence terms mostly) is something we have to deal with. Pointless forking is just a royal pain.
An outcome of this should be that I get more guarantee a ZVOL export/import cycle across Linux/BSD (which btw I have now done about 10 times on 10TB ZFS filesystems) will work with no awkward binary consequence to flag/version mismatch which came about because of two codebases: version mismatch because one was "old" and one is "new" is a different thing.