Impactions are definitely possible. But, this happens more with bread. It is often fed dry and the bird will fill up the crop (pouch in the esophagus) and then find itself unable to move it further down the alimentary tract.
Ducks especially do eat a lot of grains that they enounter in their migrations however. They will often eat a large amount at a time and store it in the crop.
Millet and sunflower seeds can be fed, but grains should be limited if possible to prevent ducks from getting fat and developing fatty livers. This is how duck pate is made.
I have never seen a seed or grain impaction in a duck however. And at least the grain is composed of smaller pieces. A few sunflower seeds might even help with digestion acting as grit. The shells might help grind the other seeds - much like pebbles.
Unfortunately, there is no way to tell you for certain. I would tell you that even though it is possible it is unlikely. Not every crop fill leads to an obstruction.
BUT, in the future, try not to feed the ducks anything. They are quite sufficient at getting their own meals. But here is a good list of things they can eat taken from a very good blog entry:
There is the link and here is the link that you can click (click here for same link as above)
Sorry that you have experienced some stress from this event. Clearly you appreciate nature and ducks in general.