"But who am I to let facts get in the way of a good story, dreamed up 150 years later, about enslaved crews singing shanties in code to fool their masters - and everybody else for the next century and a half."

Who the hell suggested that?
As for what 'they wanted to sing about' its more to do with what they DID sing about.
You will have them singing about Boney crossing the Alps wiv elefunts next ;-)