It doesn't mean much. Since it's not a developed world, the republic most likely doesn't mean much else than that there's no monarch. It's likely an oligarchy of some kind.
I mean, that's kinda what happens. An empire stretched too thin is unable to properly govern, unless they're doing city states with set rulers. You let them elect their own leaders? American Revolution. You try to keep them under force? You get multiple revolts at the same time and can't keep it together. You send out vassals to rule? one of them betrays you and fucks it all up. World domination sounds cool but holy hell does it not work. Puppet rulers get tired of their strings eventually