Northern China is a terrible economic base, I agree.

You're not fighting the same way he is, and I seriously doubt that you're attempting to compare yourself to Genghis Khan in terms of military skill.
And you attacked. So the kingdom of Babylon rose from its torpor of the past few centuries and rallied under a military leader against this new threat. None of which sounds particularly implausible to me.
I really don't see what your complaint is. Are you whining because you have to fight an NPC that isn't a pushover? None of what jalapeno_dude has done here, as the mod, is impossible or even all that improbable. And he
is the mod, so you're really not supposed to complain.
It's not statistically impossible for such a person to be born and take power in Babylon, and so long as that's the case the mod can do it.
Why does the battle need to take place far from the capital to involve outmaneuvering? Have you ever read
Vom Kriege or, hell, even an account of the life of Julian the Apostate? The concept of "defense in the heart of one's country" is a very old, tried and true method of getting your enemy to string out his supply lines and waste away along the march before smashing him. Too, it sounds as though the Babylonians were fighting at the edges of their kingdom, so they had fresh troops while Neo-Kadria had already had to fight its way through independent city-states to get there.