They're resource-poor rather than economically poor, somewhat similar to Japan during the Meiji period.
The art book says they used Chinese debt-trap diplomacy to practically buy out Rocky Island, Newfield, and use a fake nuclear accident scare to secretly turn it into their USEAN arms factory before the war. The eastern FCU was stuck in "prolonged economic stagnation" - in part due to the ISEV decimating their space industries - and the Sotoans showed up with the cash to fund "communications networks, energy infrastructure, and key ports dependent on Sotoan capital. As a result, surrounding countries, including the FCU, saw heir national sovereignty effectively hollowed out through economic dependence, leading to weakened political leverage".
None of this was conveyed in both the mini-series nor the game. The reason they hit Expo City at the start was because the UN was mustering troops there for a forced inspection of Rocky Island. These are pretty critical story details and for some unknown reason - with all the funding Bamco has poured into making this game successful - they still hide it in a pre-order exclusive book.