This is what we'd call a plot device. Eorzea has to choose. Do we believe the beastmen, put our differences aside, and wipe out the empire despite our past quarrels? Or do we believe the empire, put our differences aside, and wipe out the "false gods" despite our past quarrels?

FFXIV is based on the city states of Greece and the decisions they had to face when facing Persia. This is a way to build tension and doubt - it's good storytelling, imo.