Never attempt to deprime live primers, eventually one will go off, when it does it will detonate the others in the spent primer cup. Decapping live primers is the single most dangerous thing you can do while reloading.
Just because a person has done this practice for 50 years or thousands of times it is not safe. It only takes one accident to create a a possible injury or worse.
Whoever wrote that did so with an ambulance chaser,(lawyer), right at his side.
Take a good long look at a cartridge loaded in any gun. What is present? A chamber, a bolt or a receiver, or a recoil shield, and a firing pin poised above a primer held in a brass case. The firing pin is either spring loaded or has a hammer hit it,,,,HARD! The brass can't move it's being held by the chamber. The primer can't move, it's being held by the brass. Now it gets hit by a firing pin that has a 60# spring behind it. It detonates like it is supposed to.
Now how does a brass case with a primer SITTING in it relate to the above description? Nothing is holding that primer from being PUSHED out of that pocket. The decapping pin may look like a firing pin, but none of the conditions are present to detonate the primer. Now I suppose if some ham handed gorilla was to slam the press lever into enough primers, one could go off.
Primers have been crushed, pushed in sideways and smashed without going off. Whats missing? The quick slap of the firing pin as apposed to the slow force of an errant priming device. In other words, done slowly in a normal FL die or universal decapping die, nothing will happen,,,,other than the primer being pushed out of the brass case. And YES you can re-use them!
If the above caused you to think, instead of believing internet myth, then I apologize for ruining your morning.