Vaan had indirectly insinuated that the Celestial Kingdom was hidden to the west of the Divine Serpent Empire. Emperor Carpus was able to believe this since the blue-skinned celestial man had emerged from the west.
As such, it was easy to convince Emperor Carpus what would happen to the Divine Serpent Empire if it got caught in the middle of the war between the UN and the Celestial Kingdom.
However, Vaan didn't actually believe the Celestial Kingdom was hidden in the Western Sea.
Even if traces of the Celestial Kingdom were found in the Western Sea, it would only be a single outpost or city of that super civilization and not its entirety.
After all, at the peak of the Celestial Kingdom's prosperity, it had reached for the stars and colonized other celestial bodies in the local solar system. That alone made the Celestial Kingdom a Type-1 civilization on the Kardeshev scale.
It might have been a Type-2 civilization.
