Exactly. This "fact" gets repeated so often, but it's probably only technically true. e.g. DDG = Bing + crawl of top 500 sites that DDG does once a month
I was just taking DDG's word for it, but it wouldn't take that much to convince me that they were lying. Especially since they disappeared the page that listed all of their sources.
But not solely with Bing. DDG aggregates from many different sources, including what their own spider finds.