Their logic seems badly flawed. They study small, widely-dispersed groups - of course there is little inter-group conflict, because there is little inter-group interaction of any kind! Just like individual people spread across wide areas will rarely have sex - not because they don't want it, but because there's little opportunity if you live miles from the nearest person of the appropriate sex. As soon as groups have something to fight over - territory, scarce food, water - they'll fight unless some more powerful entity intervenes to resolve the matter peacefully.

These days, I would expect anything but the tiniest conflict between such groups to attract outside intervention, unless it somehow took place unobserved - perhaps in a remote patch of rainforest? It seems much more likely to me that they have simply noticed that current circumstances make conflict between such groups very unlikely.