A reinforcer, defined only a little informally, is an event which causes behavior to be repeated. If it discourages behavior instead, or does nothing, it is not a reinforcer. Hence, we don't necessarily know even if something which has been a reinforcer in the past will now be a reinforcer.
Kohn's message depends on the reader not knowing this technical detail, and confusing the concept of reinforcement with the concept of reward. One of Kohn's principal complaints, then, is that many of the things we call 'rewards' (gold stars etc.) are not really reinforcers at all.