I think you are confusing the notion here. The declaration was calling all Frenchmen citizens, as opposed to subjects. We're not talking about new people, but rather about how we understand the status of those already here. Citizens are equals before the law and share in control of government whereas subjects are not equal to each other (after all there's always the King) and have no control over government, with that authority vested in the monarchy. So when the Declaration called on Citizens, it was in contrast to their role as subjects. Make sense?