Interesting little research project. Milk being such a staple of the US menu makes you forget that not everyone in the world has that same advantage.
I dont think it is completely out of the grasp of reality that northern Europe in 5000 BC had a roaming ruler, or rulers, that worked similar to the Khans (~1200ad). Where he/they would bed every single woman they could in order to sew their genetic dynasty across their empire. An ancient grandpa to all of us white folk, that gifted us with his tribes genetic propensity to have lactase persistence. I feel like it is common sense that a significantly large war band would have a huge herd of cattle behind them that would feed them in that time frame. Northern tribes across that climate zone had a knack for curing meat, so they would be well fed even in the winter time.
That battle is something else though. With what researchers say, and even in that video they somewhat agree with it - the population wasnt suppose to even be that plentiful in that time period per square mile. If we go off the conventional belief, then that many men came from more than just a 100km radius.. and to me that would make a lot more sense. But on that same note, its not like we have actual proof that the population wasnt higher than what we "think" and it was these roaming war bands that decimated the population on a scale we havent seen when proportionately compared. Would be interesting to follow up on that after more research has been found.