This line from the wikipedia article about DDG somewhat contradicts you, but it is rather vague:
> DuckDuckGo's results are a compilation of "over 400" sources,[15] including Yahoo! Search BOSS, Wolfram Alpha, Bing, Yandex, its own Web crawler (the DuckDuckBot) and others.
Could you elaborate on your comment, I am sincerely interested in learning the details.