55 just restates what I said in 51. To a Roman, barbarians were those who were nomadic like the Vandals, Visigoths, and Huns. Anyone living in cities like the Parthians, Carthaginians, and Egyptians were not barbarians even if they were militarily opposed to Rome or did not follow Roman customs.
Here is the big point: At the time of the Spartans and the battle of Thermopalae, barbarian meant someone who didn't speak Greek, even a Persian who came from a civilized empire. Did the word change meaning over time? Yes, most words do.