From your source:

In fact, DuckDuck Go gets its results from over four hundred sources. These include hundreds of vertical sources delivering niche Instant Answers, DuckDuckBot (our crawler) and crowd-sourced sites (like Wikipedia, stored in our answer indexes). We also of course have more traditional links in the search results, which we also source from a variety of partners, including Oath (formerly Yahoo) and Bing.

We continue to partner with more and more sources to bring you the best Instant Answers from the best sources. Our focus is on synthesizing it all together into a superior search experience.

I suppose that's quite different than 'DDG is a frontend to Bing results'?

It depends. For me, a search engine is about the organic search results and they get all their organic search results from Bing, Oath and Yandex. The 400 sources they talk about are used for Instant Answers, boxes and stuff like that.

Essentially it's Bing for the web results. The other 399 sources seem to be for non web things.