How could a rocket system, which when in operation vents reaction mass into the vacuum of space, ever qualify as a “sealed system”? Surely only the storage tank

How could a rocket system, which when in operation vents reaction mass into the vacuum of space, ever qualify as a “sealed system”? Surely only the storage tanks could ever qualify as being a “sealed system”, and even then an imperfect one, because there must be one or more potentially leaky valves leading to the actual nozzle somehow?