Data plane management sure has traditionally been done with logical volume managers on unix particularly which introduced logical volume managers around late 80s. Not sure when deduplication exactly was available for Linux, probably around the same time as it was for ZFS on Solaris with VxVM. Various other vendors had their own LVM based dedup (NetApp comes to mind).

Anyway it wasn't really a statement about what was there first or not, but rather that because something may not exist for a filesystem does not mean it does not exist, i.e., look at other layers for such functionality.