Sorry but that is incorrect. When removing a primer you are putting pressure on the inside of the primer, not the outside where an impact can set it off. Also, since the anvil is not pressed up against the bottom of the primer pocket even if you did hit the outside of the cup it would be difficult to ignite the primer. Just think about when a primer isn't seated all the way, sometimes it won't ignite even when struck with the firing pin.
As I said above, striking the inside of the primer won't set it off. The reason we say not to slam it is, you will probably damage the anvil making the primer difficult if not impossible to fire.
Slow and steady will allow you to reuse the primer.