Campden knocks down existing yeast somewhat, but is unreliable when you want to completely stop things. Think of it more like punching the yeast in the face. It knocks them out for a while, but they wake back up. This is why it is good for use when you want to delay adding "good yeast" into something you know or think might have wild/bad yeast on it. You toss some campden to stun the existing yeast, gain yourself some time (24 to 48 hours) to do whatever else you have going on in your life, add your new/good yeast, and that good/new yeast beats out the old/stunned yeast. Like the new yeast sees the old yeast passed out from your head shot and decides to smoother the old yeast with a pillow while he is still passed out. Graphic....I know.
In this story, sorbate, if you wanted to add it, is represented by a vasectomy on the old yeast. He cant make any more babies, but if someone doesn't smoother him while he is passed out, he will wake up and still be alive and breathing. Just no new kids from him.