Welcome to BYC and the confusing world of medicating chickens! I am fortunate to have a vet who has and likes chickens; even though she has changed practices and now is further away.
She confirmed roundworms in one of my hens yesterday, and I understand your confusion. Two of my girls have a more severe case, and she gave me liquid Strongid T that I am to give once and then again two weeks later.
For the remainder of the flock, she gave me a dry version of fenbenzadole to sprinkle on their food once a day every day for five days. Then, repeat in two weeks. Based on the size of the birds in the various coops, she measured out exactly how much medicine to put in each one.
Dr. M suggested it would be better to do everyone orally. I suggested that with two dozen chickens, eight ducks and two geese, that might be better, but it would certainly be impossible. A few of my chickens would be extremely unwilling, all of the ducks would be more than unwilling, and one goose would likely beat me to a pulp with her massive wings.
IF it weren't 2 degrees outside, she also said putting the dewormer in the water would work. However, I'm already going outside three times a day to replace frozen waterers, so we crossed that off the list since most of the medicine would be thrown out with the ice.
When I asked about egg withdrawal, she said the U.S. government said 30 days. I asked what would happen if I ate an egg during the time the birds were being medicated; she said fenbenzadole is used to treat worms in humans and let me draw my own conclusion.
Good luck with getting rid of the worms!