Comment by awused

In my example the sum of possible future allocations for ZFS is still only 10GB total. Each of the ten file systems, considered individually, does truthfully have 10GB available to it before any data is written. The difference is that with over-provisioning (like LVM+XFS), if I write 10GB of data to one file system the others will still report 10GB of free space, but with ZFS or BTRFS they'll report 0GB available, so I can never actually attempt to allocate 100GB of data.

You could build a pool-aware version of DF that reflects this, by grouping file systems in a pool together and reporting that the pool has 10GB available. But frankly there's not enough benefit to doing that because people with storage pools already understand summing up all the available space from df's output is not meaningful. Tools like zpool list and BTRFS's df equivalent already correctly report the total free space in the pool.