Alectai
Speaks Words of Wisdom... On occasion
It's... NOT a fact for hunter gatherer nomads though. It's a thing for pastoralists. With hunter gatherers the costs are high and the rewards low, because everything another tribe has is on that tribe's persons, and hunter gatherers do not generally possess much in the way of surplus. With pastoralists the costs are much lower, because a small number of people can look after a large herd, and the rewards are high, because herds are extremely mobile.
You're forgetting the climate issues, the further South you go, the less grazeland and the more hostile the environment is. The strongest tribes get to monopolize the best grazeland, and everyone else has to make do with what's left over. The White Sky's arrival causes a massive paradigm shift because they can establish safe enclaves that don't require your kids to freeze to death and can maintain a semblance of feed for your herds.
Like, that's the thing about nomadic societies, they're not that way because it's an optimal choice--they're that way because they tend to get outcompeted in peacetime by agricultural societies (Which is why they do so much of the raiding to make up the difference).
As far as the Cloud Tribes are concerned, survival is a zero sum game where your prosperity innately comes at the expense of your neighbours. They just project that forward onto other societies as well as a general rule even when it doesn't apply.