I have mixed feelings about this. If you recommend DuckDuckGo, Qwant, Ecosia you're essentially recommending Bing. Each of them have to show bing ads, forward the users' IPs as per Bing's terms of service (to fight ad fraud etc). Similarly, if you recommend StartPage, you're recommending Google. Same drill with ads. I like DuckDuckGo. But unless they build their own search - I think the fundamental problem has not be…
It's not mentioned on their site, or their wikipedia entry, or anywhere else I could find, and the results were different when I tried a few searches in them both just now.
Are they "Using Bing" in the same sense they "Use Wikipedia" to put info boxes on the results page sometimes?