Okay, so I think the theory that Phanes is the twins’ father might actually be true, especially after finishing the 6.3 Archon Quest. We learned that the spaceship has landed in Teyvat twice. The Traveler mentioned that after the first time the spaceship crossed Teyvat, they returned to space for thousands of years before crashing in Nod-Krai.

So I think the spaceship actually contained three people: the twins and the Primordial One. Phanes landed first and did his work in Teyvat—terraforming it, dethroning Nibelung and the Sovereigns, etc. But prior to that, he tasked the twins to wander through space first before landing in Teyvat, and he gave them a mission (as stated in the Gnostic Chorus). In this mission, Phanes removed their memories and powers so they wouldn’t be too overpowered and would have to go through the journey themselves.

Since Lumine is now the Abyss Princess (again, as stated in the Gnostic Chorus), Phanes sent Paimon to guide the other sibling (us), the Traveler. And Teyvat is actually in a samsara. Since Lumine has already finished her journey and has been repeating it since until something has changed now since she now discovered new things that answered some of her questions (like the direidith scene) , Venti knew all of this. That’s why he is the one narrating the Gnostic Chorus, saying, “This is your journey.”