It's just that, a theory. But if anything should be a clue, it was the wall scroll they had in the background on the Live Letter from the Producer.

Other than that, I mean, what ELSE could really take on Bahamut on that level? Louisoix was done, the Twelve were done, the Garleans weren't going to intervene. The beastmen sure as hell had a say in the future of their own races as well, so we shouldn't forget that they too could have had tricks up their sleeves.