But that's exactly what I mean: ZFS snapshots make copy-on-write branches of a single volume. Time Machine backs up files to an external disk. Unless there's some proposal to make Time Machine-managed external volumes be part of a pool that's only attached some of the time and thus get some ZFS benefit from that, I don't see how TM would be any different in a ZFS world.
Now, local snapshots would be useful for sure (and could be integrated into the TM system in various ways), but they're totally orthogonal to separate-disk backup/versioning systems like TM.
It would not be just different - it would be much better.
With ZFS, Time Machine would not require an extra disk and could maintain nearly continuous history of the whole file system even in single disk configurations. The minimum time between snapshots could be easily adjusted so that not much disk space is consumed in the process.
To say it would be "much better" ignores that one of the primary usages of Time Machine is that it provides easy whole-disk backup in case of catastrophe. In this scenario (the primary reason why many people use it), ZFS is completely irrelevant.
Now, perhaps one could imagine a future version of Time Machine that would read from source file system snapshots while making backups to the external target in order to capture versions that were made while the user was unplugged from the external disk; and perhaps ZFS would be appropriate for Time Machine target disks in order to save space via block copy-on-write. I'd say I expect both of these features at SOME point in the future ("expect" inasmuch as they're obvious wins and one has to think that the future direction of file systems is to include constant time low cost snapshots via copy-on-write), though I don't know if the file system(s) will actually be ZFS-derived or not.
But this is a far cry from saying that Time Machine functionality would be free on top of ZFS. Any future version of Time Machine will include replication to an external disk (or, at least, "the Cloud") and that's NOT a feature of ZFS as it stands right now.
Now, local snapshots would be useful for sure (and could be integrated into the TM system in various ways), but they're totally orthogonal to separate-disk backup/versioning systems like TM.