Those people did not know what they were doing.... (Indian Runners are renowed to be great to train herding dogs...!)
Not long ago, I have been contacted by some people who wanted to buy some of my Indian Runner drakes to train their herding dogs...
...They then asked me to sell them my ducks at below-cost price... or even to give them for free if possible : given the ducks were at risk of dying during the (dog) training, they (these people) really did not want to put their money in the ducks.
I've seen it done right. It starts with training the dogs without live animals until they are reliable. Then having the ducks in a big playpen type area with the dogs outside it. The dogs learn to listen to the directions of the people even with the ducks close. By the time the fences are gone, the dogs are listening to the people - not endangering the ducks.
Done right, the dogs never chase or catch the ducks. They move at the phycological pivot points so that the ducks drift slowly in the desired direction.