Again, war, him annihilating the enemy is not invalid, especially because again, he doesn't know what they're capable of, you take him as unmatched because you as the reader know more than Ignis and the humans, but in-universe Ignis doesn't know what they can do, so he's not taking any chances.
Irrelevant if they're overpowered, because again, war, as they say, fuck around and find out.
And no, there's no need for an all out war, they attacked first, they suffer the consequences.
You don't get to start a war and then whine when you get your ass kicked.
Just because one side is the "good guys" it doesn't mean they need to play "nice", this is fucking war, war is ugly and brutal even when the sides are equal, why would it be diferent just because one side is stronger?