Some things are explained by the game, for example, if there was suddenly a Gatling Gun developed, we already have an alchemist named Gatling in the game to have developed it. But we've yet to see any evidence of a Viking (or Viking-style) civilization in Eorzea to have developed and wielded Viking Swords. If there's actually a Viking in older Final Fantasy games that could come up in XIV, I wouldn't know. But chances are they're just using it as a name in the game. I'd avoid reading too much into the usage of the word "Sipahi" in much the same way.
