There are enough TKD places that do this type of sparring...I'm going to apply that counter-argument here. If we have to make our own unofficial club and practice something on our own because we don't do it in class, we can't argue that it's part of the system. If I went to a boxing gym and unofficially taught people how to throw a spinning hook kick, you could hardly say that the spinning hook kick is part of boxing.
So, lets talk boxing: https://www.ianstreetz.com/single-p...es-7-The-Crouching-Crab-Crab-and-Philly-Shell
One gym may teach the Philly Shell only. Another may teach the Crouching Crab only. They are both teaching boxing, just with different views. A Philly Shell student, that went outside of his gym to learn the Crouching Crab, because they don't teach it in his gym... is still studying boxing, even though the Crouching Crab is not part of what they teach in his gym. This student will have not only a better knowledge of boxing... but he will have a better understanding of the Philly Shell. Learning that other style will show him the holes in the Philly Shell and how to attack them. It will also force him to learn why things in the Philly Shell are done that way... things he never would have though, had he not ventured out to try the Crab.