I would not be surprised if one could write a ZFS implementation optimized for more constrained devices. If you already know you are going to a have flash storage you can probably ditch some of the N layers of cache you see in common ZFS implementations. Not that ZFS is a one size fits all, but the file systems specification could be implemented in more than one way.
If you assume Apple cares about having a disk format in common with other platforms, sure, I'd agree that's probably possible. But I don't think they do; they seem to care a lot more about things like a unified codebase across their platforms, the energy-efficiency initiatives they've been pushing for a few years, owning the key tech in the products, etc.
One slide in the WWDC talk deck showed a bunch of divergent Apple storage technologies across all their platforms that are being replaced by APFS. If ZFS has to fork into weird variants to run well on the phone or watch, that seems less appealing than a single codebase optimized for just the stuff Apple products do.
