They're using prince to mean the ruler of a domain (as in Machiavelli's The Prince), not the heir to a land.
From Latin
Princeps, metaphorically "First Citizen". The use of Prince as a junior rank for a King's son is solely British, IIRC.
OFF TOPIC: Boethiah is Greek, roughly "Help me!" and was screamed by the man who tried (and failed) to stick a dagger in Caesar's briast solo. His cry summoned the rest of his conspirators (Cassius and Brutus among them) and a debauched stabbing-party commenced. It's likely that this was intentional on Beth's part.
EVEN MORE OFF TOPIC: The full quote was
Adelphe, Boethiah! (even more roughly "Help me, Bros!")