I didn't read the article but the headline makes me postulate that this is because the optimal individual strategy in Prisoner's Dilemma is tit-for-tat.

Respond in kind to every action taken against you. If someone offends, attack, if they offer peace, be peaceful.

Nice people will more often offer the olive branch (sub-optimally), so anyone else playing the optimal strategy will offer it back. Over time this probably results in a lower violence rate. Even mean people (also sub-optimal) will get dragged into niceness over time since nice/mean probably changes dynamically.