Even though it was a "when ready" descent, once you start the descent, technically you shouldn't descend at any point at less than 500ftpm. So in this situation you are correct- it should have been a continuous descent to FL210, but making sure you made the level restriction stated. Sure somebody's going to disagree with me though!
On the other hand, the controller has given the descent restriction for a reason, so as long as you made that restriction, the controller probably wouldn't have been too bothered if you levelled of at FL240...