I'm pretty sure APFS isn't Apple's reimplementation of ZFS.
As impressed as I am with ZFS on a server or similar situation, there's no way ZFS was going to be the solution for a watch, set top box, headset or phone the way APFS is.
The vendor-neutrality is the main thing. There was a point in time right around Mac OS snow leopard where the Unix world would have a common filesystem. This would have been for the common good. Sadly, lawyers got involved. Shakespeare was right.
As impressed as I am with ZFS on a server or similar situation, there's no way ZFS was going to be the solution for a watch, set top box, headset or phone the way APFS is.