Was Gaven bluffing?
Based on their understanding of Gaven over this period, not only was he not bluffing, but it was even probable that his descriptions were modest. The annual outputs of these two cities could be higher than his estimates, as they were in a continuous state of development. This Monster Tribe, in terms of recruiting population, had advantages that humans did not possess.
Wealth moves hearts.
Such terrifying outputs, once exposed, would certainly attract countless covetous eyes.
Especially for cities like those on the Giant Dragon Coast and the Dark Forces that were driven by money might ally again to march towards the Giant Plains and those two cities, which was not an impossibility.
Having a "nail" secured here would indeed be very beneficial for them, and if they could use this opportunity to expand and grow stronger, then that would be even more favorable.