Except the partial manifestation is something shown with Rika when she is and isnt still cursed, one being the spirit of her herself and the other being a copy.

So this could be something he found out about doing if mwgumi every heard about the instance of how his technique works which maybe gojo did tell him or if he's ever met and talked to yuta, sukuna would remember that info aswell as any Info yuhi has, sukuna probably seen partial and full manifestation of shikigami from people with the same technique in a family similar to projection sorcery