A warmer planet is a greener planet with longer growing seasons, more liquid water, and a greater abundance of flora and fauna generally. I don’t think this is in dispute. So their theory of increased competition over dwindling resources would have to be based on a time of cooling, not warming, yet I have a feeling that if I read the article I’d find they’re claiming that warming was the culprit.
It may be that with the warming 13,000 years ago some were able to adapt more quickly to growing crops and forming civilizations. The crops and “wealthy” villages may have looked real inviting to those stuck in their hunter-gatherer modes of living.
So instead of hunting the dwindling large ice-age mammals, they preyed on the farmers and their supplies.