The whole economy system of the world seems to be fucked. Demographic booms have never happened during wars, but after. Think of it this way: mr soldier goes to war, he marries the girl he loves before leaving, then he hurries off; the years pass, mr soldier survives and comes back. What is the one thing mr soldier is going to do with the wife he hasn't seen in years? Yeah, so that's where the word Boomer comes from. Even if the Southern Kingdoms had the other kingdoms share some serious medical and agricultural knowledge with them, the population would still be barely increasing, because that's what mobilizing your armed forces does. I think they are mobilizing because the hero mentioned that the geography isn't very suitable for agriculture, and that most of the supplies come from the North.
Speaking of which, what the fuck is up with that market shit? Feudal societies would just buy the weapons and supplies from their producers and merchants, then send them to the boys and gals at the front. Why bother with sending money, then the supplies? Why aren't they contributing troops and shit? Yeah, supplies are all well and good, but who's gonna use them? Overall, the author seems to have gotten lost themselves in the details. Hopefully all will make sense later.