I guess until they've done it, they're operating on theory. At least their makers seem to have predicted some of the attributes of the promised world. That's a good indication they have at least some idea what's up.
What if everything is just one grand rationalisation to justify their hedonistic and genocidal ways?
How could they know from before about a promised world, especially when they have tried the experiment (presumably) other times and they failed? All we have is a forgotten prophecy of a species that has been driven to the edge of annihilation in charge of technology they barely understand.
It is just a projection to justify their depraved nature which has been hollowed out by the unattainable pursuit of the absolute of salvation. There is no salvation, there is no justice, just the gods eating the souls of the living.
I can only assume their understanding of metaphysics has given the progenitors reason to believe that such a thing exists. At least some of their predictions have born out.
Plus, y'know, not much of a theme to the series otherwise.
My reading is that all efforts to achieve the great themes of the series, such as pursuit of the Absolute, Salvation, following Heroic leaders, living and dying by the Book fail and quite possibly have always been in vain.
The Inchoroi drove themselves to extinction in their quest to reach salvation.
The Dunyain despite their efforts to master all circumstances, were destroyed by ancient aliens who used magic and genetically engineered beasts (two possibilities that the Logos could not predict) and then the last survivors of the Order succumbed to an even greater evil.
Kellhus despite his meta-gnosis and his infernal pact with Ajoklis is undone in the moment of his triumph. And we don't even know if his plan would have worked. In any case the TTT couldn't take him beyond the Golden Room. So even TTT fails and subsequently failed Kellhus.
Proyas lived his whole life trying to be pious, and we know how that turned out.
Achamian the scholar and the sceptic, despite his hatred for Kellhus in the end sort of believes that maybe the Aspect-Emperor had plan. But if Achamian was to find out what actually took place in the Golden Room his assessment for Kellhus could easily be revised to something negative again.
Esmenet the only person who may be holy, as per the Judging Eye, has given birth to a monstrous family and her husband was plotting to become the Avatar of the God of Deceit. It doesn't get more ironic than this.
All grand designs come to nothing, nothing goes according to plan.
I suspect that the Second Apocalypse, which I guess will the subject matter of the third series, will fail to seal off the world. Which then leads back to the original question, if no world was ever successfully shut, why do these crazy aliens keep attempting the same genocidal action every time? Isn't the definition of madness to keep doing the same thing but expecting a different outcome?