Yes, I think it is something like that. As we see in Book 2, priests etc are called illuminaries…avatars have illuminaries that feed them mana they harvest from worshipers etc. Illuminaries are like pipes, the more pipes you have on a plane, the more mana you have locally to draw on without having to pull it from distant places.
I’m thinking it has to basically be a logistics problem, i.e. a mana supply line/train etc.