It's a hack, but it's also the best (only?) way to deduplicate all that metadata without breaking the ability to access the history using pre-existing filesystem operations.
Another way, which is worse, is block-level deduplication. All of the above filesystems support it, as does NTFS.
I wish Apple would adopt HAMMER for Mac OS. It is BSD-licensed and more suitable for a memory-constrained environment than ZFS.