That's sort of right. Rev Matching is really difficult to get from the start because it all depends on what RPM you are shifting down from. For me, driving a race car rev matching is simply heel toeing. Which is basically only done under braking. I've never really found rev matching useful while maintaining a constant speed. With a sequential car and a clutch pedal you have to; Clutch in, push gear lever forward to downshift, rev engine at speed with clutch, clutch pedal release. But that's really only for braking. Double clutching is more of a rev matching while maintaining the same speed sort of thing. Clutch in, neutral, clutch out, little bit of throttle, clutch in, down a gear, clutch out, all with a little bit of throttle input. If you do it correctly the car won't jerk or anything. You won't even really feel the car weight transfer or anything.
From one sense it is damaging the clutch but from another it's not. If you are driving at higher revs and downshift without applying throttle that's going to basically cause the clutch to grip really hard which would hurt the friction plates. You kind of have to know a lot about cars to understand what would be better for each situation. Like it's impossible to explain.