Well, no, the feed store guy isn't quite accurate with the information on medicated chick feed and preventing or treating coccidiosis.
The purpose of the medicated chick feed which has a very low concentration of amprolium, is to help a chick's body develop a resistance to coccidia. If your weather contributes to a sudden explosion of coccidia populations, especially before chicks develop a solid resistance, they can get sick from them in spite of the medicated feed.
The same is true if the coccidia explosion overwhelms the chicks' resistance, even if they've developed some. This can happen to adult chickens, too, even though they've been resistant to their local strain of coccidia for years. Once the population reaches a tipping point, they can cause even the healthiest chickens to get sick.
Coccidiosis is one illness that is simple enough to treat successfully when caught early. More often than not, people will treat for it when they have even the slightest suspicion it may be the cause of their flock being sick. Time is of the essence in treating it before it causes irreversible damage to the intestines and secondary bacterial infection that will require a special sulfa antibiotic to cure, if it's not already too late.
I would like to stress, if you are torn between trusting the information and advice you get from a feed store employee and what you get from the experienced old hands here, I urge you to choose the advice you get here.