Even in our world, the prevalence of technology is never even. Gun-equipped European soldiers won their colonies by beating natives armed with bows and spears. Brits used calvary charges against machine guns and cannons in WW1. Germans and Soviets moved supplies and artillery around Europe via horse-drawn carriages in WW2.

Right now Eorzea varies between medieval to renaissance to industrial era tech depending on where you are. Limsa Lominsa has rifles and cannons and ships powered with ceruleum engines while Gridania still fights with bows and spears. All cities are interconnected by a network of airships and even Ishgard builds their own and has gunpowder-propelled harpoons.

Since the world is in technological upheaval as advancements and Garlean tech leaking out spreads everywhere, it’s not going to be even and we’ll still see dirty medieval peasants in one place while the next town has electrical lighting and airships.