Looks like they built a new NAS, but kept using the same drives. Which given the number of drive bays in the NAS probably make up a large majority of the overall cost in something like this.

Edit: reading comprehension fail - they bought drives earlier, at an unspecified price, but they weren't from the old NAS - I agree, when lifetimes of drives are measures in decades and huge amounts of tbw it seems pretty silly to buy new ones every time.