I think the whole sentence is a bad take. The described behavior can be perfectly rational (and thus commonly considered not "stupid") in the case when cost function of the acting person has a negative weight assigned to the counterpart group/person. In other words, when someone considers the other an "enemy", it makes sense to hurt the other even such act results in some direct losses.

Now, we can argue that playing negative-sum games is "stupid". And in most contexts of the modern human society such heuristic would be correct, but I would be really careful with a sweeping generalization, otherwise instead of a proper understanding of the underlying behavioral motivations you are likely to devolve into primitive explanations of someone being "stupid" or even "evil".