There were a few other verbatim quotes from mythology.
Come back with your shield or on it"
Also it wasn't really based on history.
First there was a historical event. Then there was as bad historian who turned it into legend (hence the mutated Persians). Then the story was hollywoodised movie 300 Spartans. Then it was Frank Millarized in 300. Then it was hollwoodised again in the movie 300. (And there was Meet the Spartans but we don't like to talk about that.)
In other words, the movie 300 has much to do with the actual battle as the Homestead Strike.
Also I think Frank Miller is usually ironic/comedic/satiric in his books. In fact I can't think and serious stuff he did other than Daredevil and Dark Night Returns.
Also I though Dark Knight Returns wasn't conservative because they portrayed Reagan in a very negative light. Then again it's hard to present him a good light with a straight face.