Feed is a replacement for the more or less balanced diet that ducks eat in nature. That's how animal feeds work. You give your ducks duck feed instead of going out into nature to pick up water plants, insects, small fish, and everything else they'd normally eat.
If what you're using as treats is highly nutritious, then it becomes part of their normal diet, and doesn't really fit into the "treats" category. Some people feed a processed feed as most if not all of the diet, some people use feed as a part of the diet and also give other things. If you're giving over 50% "treats" and the ducks are healthy, that's only treats in a sense of the ducks enjoying it. If that makes any sense.
I'd be wary of spreading the idea that you can feed any animal mostly on kitchen scraps, though. It depends a lot on what the household eats, and you have to watch closely for any deficiencies. All animals need a balanced diet, and some households wouldn't put out balanced scraps.
To rephrase, when people say 5-10% treats, they mean 5-10% foods that are especially tasty but may not have the most nutritional value, or may be badly unbalanced. Egg is a great treat, but if you feed a duck 75% egg and 25% feed, you are not gonna have a happy duck. For the purposes of the above, by "treat", I mean one of those tasty-but-not-nutritious foods. It's also possible to keep some animals healthy on a diet containing a lot of what they would consider treats, but with a lot of nutrition.
(I'm up way too late, hopefully this is coherent. Basically, it depends on the treats.)