I have a feeling zetsus recording ability ties in somehow with Sasuke and Oro's current trip to find the ones who know everything.
Sasuke's at least seen zetsu before, theres technically several of them, but more specifically the Black/White zetsu would be 2 people but share one body.
As we all know he is of the earth itself, which means he could have existed before madara gave him a physical form (kind of like father from fma) but only
after harnessing his skill to make the bodies extracted the information from the earth via hashirama/gedo mazo. it would explain how he mysteriously put all
of these ancient facts together for the moons eye and the unknowns we've yet to see sasuke meet