you dont need DO in Hempy buckets because the nutes arent totally in nutrient solution, they are in medium, this medium contains air, therefore air get to roots via diffusion because medium contains air, last two inches contains nute solution that is constantly being taken up the plant via a mechanism known as the transpiration stream, therefore constant movement of nute solution = no stagnant water. when a plant is dry and goes floppy it needs water because turgidity is the mechansim that keeps a plant stiff.
A DWC needs moving water so DO occurs in the nute solution so roots get oxygen via diffusion otherwise if the water was still the roots would use all that o2 in no time causing anaerobic respiration.
Hope this helps.
You still need DO in any hydro including Hempy. I got root rot once in hempy from low D.O.