To be clear, BTRFS also supports in-place upgrade. It's not a uniquely Apple feature; any copy-on-write filesystem with flexibility as to where data is located

To be clear, BTRFS also supports in-place upgrade. It's not a uniquely Apple feature; any copy-on-write filesystem with flexibility as to where data is located can be made to fit inside of the free blocks of another filesystem. Once you can do that, then you can do test runs[0] of the filesystem upgrade before committing to wiping the superblock. I don't know for certain if they could have done it with ZFS; but I can imagine it would at least been doable with some Apple extensions that would only have to exist during test / upgrade time. [0] Part of why the APFS upgrade was so flawless was that Apple had done a test upgrade in a prior iOS update. They'd run the updater, log any errors, and then revert the upgrade and ship the error log back to Apple for analysis.