I think the netherbrain already dominates other elder brains in the multiverse. Where else would all these nautiloids come from in the end ? Certainly not from the colony under moonrise towers. And Omeluum said the knowledge to build them was lost, so these must be very old, precious artefacts coming from wide and far and not easily turned over by their owner brains..... Although the easy answer would be, another inconsistency in Larian's storybook.
