That’s technically true, but it’s as misleading as saying they get their search results from Yandex. Their results are aggregated from several search engines, not just Bing. They also have their own web crawler, DuckDuckBot, which absolutely respects RobotRules.
Edit: I’m told my information is out of date. No more Yandex because of Uncle Sam. Yahoo is just Bing now, so that index doesn’t count anymore. The bulk of the rest of their sources are largely inconsequential specialized search engines. Their sources page states that they “largely source from Bing”.