This point about Diplomacy culture illustrates one of the most important ways in which it's not particularly true that

The conditions of Diplomacy - competition for scarce resources, rational self-interested actors, importance of coalitions, lack of external enforcement mechanisms - mirror the conditions of game theoretic situations

as stated in the OP (emphases added). The players aren't self-interested because, in the usual case, they're playing with friends and acquaintances. (Or even simply because they're typical human beings interacting with other human beings.) And the availability of external enforcement mechanisms has already been pointed out.

Of course, game theory doesn't actually require self-interested actors either. At least not in any sense of "rational self-interest" which goes beyond "rational interest" or, for brevity, "rationality".