Perhaps this will help. If by "medicated" they mean with amprolium, it is simply something that any cocci present will take up and which will inhibit their uptake of thiamine. This causes the number of cocci present to decrease but not completely disappear. Chickens naturally build up an immunity to cocci, but for chicks, if they are present in large numbers, the chicks can become overwhelmed and get sick or die.
Meanwhile, the chckens don't really absorb the amprolium, so it doesn't afect any eggs being laid, or cause thiamine problems for the chickens.
Cocci can quickly overwhelm a young chick and kill it before you can get to the store for its primary treatment, Corid -- which is simply a much more highly concentrated form of amprolium.
Some soils have a much higher concentrataion of cocci than others. I've fed medicated to mine, but I don't know that there are a lot of cocci here, at all. For all I know, I could have fed nonmedicated. In some locations, you may have to increase the dose of the Corid. Some members give a preventive dose of Corid to their chicks, as they have found the medicated feed isn't sufficient. In some cases, the Corid isn't enough and people have to give a praticular sulfa drug, whch in itself is much rougher on the chicks than Corid.
This is hardly an exhaustive discussion of the subject, but anyway, I'll also mention that there is now a viaccine for cocci. It is fairly new and I'm afraid I know next to nothing about it, other than some hatcheries offer it, and they recommend that vaccinated chicks receive unmedicated chick feed.
