MSQ Blue moon (spoilers if your not at 3.5 do not click this)

[alimdia]

If you read all the dialogue in the MSQ, you'd find that they did indeed say he sealed himself inside with the primal.

The spell swallowed him the same way the blade of light swallowed Moenbryda, Papalymo is gone, he's not sealed, his life aether was spent creating the cocoon.

The sealing spell is extremely aether costly, for the calamity Louisoux had to use the prayers of Eorzeans to the twelve to gain enough power for the sealing spell, he basically started a primal summoning using the prayers of the twelve to channel their power through tupsimati, for Baelsar's wall we didn't have that power readily available, so Papalymo uses the remaining power of Tupsimati and his own aether to finish the spell.