> Surely DuckDuckGo has implemented their own search engine, crawler and indexer apart from other providers?
No. They have their own crawler but it's only used for Instant Answers and other widget stuff.
https://duck.co/help/results/sources
> In fact, DuckDuckGo 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.
So in other words, all organic search results come from Bing and Oath.