It’s “that you can only have one item crafted as Primal from the cube”.
It’s a general limitation on those items – I guess the idea was that these upgraded primal were a temporary measure – until you find the same item with as good (or better) stats either primal or ancient. Then you swap them, upgrade another item to primal, and farm for that item. Or something like that. 
See 2nd paragraph of the blog for Season 28:
