The sustainability of tit-for-tat and cooperative strategies as equilibria in repeated games based on discount factors is a foundational result of game theory (the Folk Theorems) and does not require citation.
The claim refers to a standard, foundational mathematical result in game theory (the Folk Theorems regarding repeated games and discount factors). As a textbook definition and core tenet of game-theoretic analysis, it is common knowledge within the discipline and requires no empirical citation.