I only skimmed the article, but I'm pretty sure that tit-for-tat as a strategy is not the relevant point. The point is that there are people whose strategy is always-cooperate, and enough of those players in a group results in a situation that will stabilize to having a decent amount of real cooperation. Tit-for-tat might have some similar results but probably would need a greater number of tit-for-tat players for stability versus the critical mass of always-cooperate players needed for stability (my speculation).