The lore implications of this story are massive. Here's some speculatory rambling:

I bet they managed to take advantage of these souls that were able to fight against the lifestream in order to summon Zodiark. The Zodiark statues around The Words of Lahabrea already make it seem like they had been working on a vessel; with half the population willing to sacrifice their lives with great determination, I'm sure the souls could have resisted the pull of the Underworld and remained in the star's vessel just like the one trapped in the firebird.

We can see Emet still possesses the power described here when he just snaps his fingers to pull Yshtola out of the lifestream in Raktika. "Essence and apparence", it seems he sees both the normal and aetherial versions of the world at once.

Hythlodaeus being Emet's equal is also interesting. Maybe this lends credence to the theory that he was the disgraced 14th member of the convocation who summoned Hydaelyn, and that the WoL is a reincarnation of him.

Varis's obsession to "perfect" humanity may have been rooted in that line.

God if he has dreams like this whenever he goes to sleep, it's no wonder he takes hundred year naps all the time. He just wants to reexperience what life was like with his bros before it all shattered.