It was always my understanding that the feeding call is used not to say to ducks "we're feeding" but to reproduce the actual sounds of ducks on the water in a group.

As your post states this would be a normal sound or communication that passing ducks would be accustomed to hearing. They do also make the feeding chuckle in flight.

Think they just call it the "feeding call" because it generally occurs when ducks are on the water and are in fact feeding. And it also sounds better than the "get the hell away from me" call.

Going to local pond or marsh is best place to hear actual calls in action.