If the country of rationalists is attacked by a country of barbarians who are perfectly optimized for conquest, the rationalists will get conquered.

But there's no way to get from here to there except by Omega coming down and constructing exactly the race of barbarians necessary for the hypothetical to work. And if you're going to say that, there's no point in referring to them as barbarians and describing their actions in terms like "believes in an afterlife" and "obeys orders" that bring to mind real-life human cultures; you may as well say that Omega is just manipulating each individual barbarian like a player micromanaging a video game and causing him to act in exactly the way necessary for the conquest to work best.

Except of course that if you say "rationalists could be conquered by a set of drones micromanaged by Omega", without pretending that you're discussing a real-world situation, most people (assuming they know what you're talking about) would reply "so what?"