Could we imply that Exogol is somehow a kind of Star-Forge that has frozen over the ages into a planet? It would be a good explanation for why the destroyers suddenly emerge from the under the ice as well as expand some of the lore of the film. Perhaps the huge pyramid like structure has an important role in the film other than just being there for eye candy. Maybe it could written in the crawl that Kylo, after being tormented by voices from the beyond, heralding his doom, has for the past years taken on a galactic wide conquest for an ancient weapon of the Sith. Capable of conjuring battle ships, the quantity of which could invade a thousand worlds.