You’re certainly correct that in-group cooperation is present in hunter-gatherer societies; my point was that it’s rarely seen in group-group interactions where zero sum thinking is nearly always dominant. Evolutionary theory predicts both scenarios. Within small tribes/groups/clans there’s almost always close genetic kinship so reciprocal altruism is highly adaptive. Conversely as there is rarely close genetic kinship between different groups we’d expect to see zero-sum type interactions and that is, for the most part, exactly what we do see.
One of the tremendously powerful things about evolutionary theory is the fact it can explain so much about human behavior (as well as, of course, the behavior of all other living things) in ways that can be empirically determined, rather than leaving us to rely on unsupported abstract theorizing or personal beliefs. It’s why I wish evolutionary theory was taught properly in schools and universities.