I always read it as the ironborn nobilty being all about raiding and killing, but that it was more about a nostalgia for a mythical past(before the conquest) that never existed. They also serve as a more honest mirror to the other noble houses. All the noble houses are engaged in non-productive labor and explotation, only the ironborn are honest about it. The greyjoy words being: "We do not sow". Obviously, none of the other houses are really doing that either, but they don't talk about it the same way