The battle of Thermopylae was a victory, just not a physical one. For three whole days, the 300 Spartans held at bay 30,000. In addition, they faced differing tactics and units from all over the known world. Darrius should have marched right through them without missing a step. So pyrrhic victory or not, Leonidas did in fact give "The God-King" a butt-whuppin.
As was stated ad nauseum in the film 300, it was not about a physical victory. It was about making "a god-king bleed". Sure, Darius would in fact go on to conquer most of Greece, as if there was any doubt, for the rest of his days, in the back of his mind, he never forgot the "bloody nose" that Leonidas gave him.
Back on topic, while Firaxis does what it can for accuracy in their games, they are far from being any kind of scholastic authority. That and accuracy can be a subjective term when dealing with historical facts. Talk with a Christian about the Crusades and you will get a very different answer from some of it's victims.
I think Firaxis should have stuck with the Old Persian for continuity and the Persian empire spoke it before they were conquered.