Sure! If you get hit hard enough, you die. No one is claiming you can jump off a bloody cliff and survive if you are wearing cuendillar armor.

A bullet proof vest can prevent a bullet from killing you. The bullet hits the vest. It hurts a lot! but it doesnt kill you. because the force of the bullet is disapated over the surface of the vest.

Lets look at Perrin's hammer for instance. If his hammer hit your skin directly then all the energy of his swinging hammer is released in the small area his hammer is hitting you.

Now, put a ANYTHING between you and the swinging hammer. If the surface area of that THING is smaller (IE a nail) then it focuses the energy of the hammer onto the smaller surface.

If the surface area of that THING is larger (IE cuendillar armor) then it spreads the energy of the hammer onto the larger surface area therefore doing less damage.

This is a very simple illustation and there are other things that come into play such as different materials absorbing energy better than others. Cuendillar being one of the better materials for absorbing energy.