Healthier doesn't necessarily mean more powerful. The North American indians living by the chase were absurdly healthy compared to European immigrants. That didn't stop them losing out when push came to shove, because their weaponry was less sophisticated than the invaders - and, importantly, because there were fewer of them.
But this article is claiming that milk - not cereals - is what gave the incomers the edge. Over-use of cereals will result in population growth (but accompanied by a decrease in average stature) - because they provide cheap and (relatively) reliable calories. A larger population means more fighters even if they're smaller. However, the use of dairy products might result in population growth but not necessarily a diminution in stature.
However, the Der Spiegel piece is a bit speculative anyway. The lactose-intolerance stuff is a bit of a canard, so I'd be disinclined to hang too much on it. You can make cheese or yoghurt with milk; you don't have to drink it "as is". There also seems to be a certain amount of evidence that some people who are supposed to be "lactose intolerant" can tolerate at least some raw (as opposed to pasteurized) milk.