In a single act, such as a battle, large groups of individuals with roughly the same genetic composition are reduced overall. The winners perpetuate, without clear indication that a genetic difference led the Greeks to success or that a genetic difference can be observed in the population after the battle.
There are schools of social Darwinism that, of course, propose that human behavior can be explained in Darwinian terms. The success of a war is dependent on a variety of factors which are not selectable, for example, the hatred between the Greeks and the Persians. What it comes down to, really, is that the ability to wage war at all can be selected for in Darwinian terms. Both Herodotus and the Persians had the gods on their side.
Yet any break from this exclusive use does not commit us to a theory of social Darwinism, even if some have tried to apply the principles of genetic selection to human choice and the complex context of human praxeology. I give the comparison simply to point out that the attribution of success requires more than pointing out the single act of a de facto survivor. Measures of sufficiency, fitness value, and the success of adaptive changes in terms of functionality does not give much new information beyond the mere change that has been observed. In light of that, I find some sympathy for the smart aleck who replied, "duh!" although I do hope the poster has read his Hume.