First of all, I never claimed that DDG results were exactly the same as Bing's. I said the top results were the same. Admittedly, I used less random terms than you did.
However, I did try your experiment, same terms. The top results from DDG were indeed found in the top results of both Yahoo and Bing. You are right though, not in the same exact order. Then I tried the same terms on Google. There is only one shared result on the front page (the Nasa chief one). I went through 4 more pages of Google, none of the results that both DDG, Yahoo, and Bing all seem to share were on any of them. This aligns more with the point I was trying to make:
DDG is going to have a hard time improving their results if the information they're getting is from inferior sources to begin with. It's like working on incomplete information. No matter how good their algorithm is that ranks and filters results from other search engines, if those other engines suck, there is no way their results can be that much better. If, for a given term, Yahoo returns site A, B and C, and Bing returns Site D, E and F, is there a way for DDG to determine that actually site G is the better result? The results can't appear out of thin air.
Also, you claim with emphasis, that the only weakness of sourcing from third parties is the indexing problem. That's absurd.
Obviously, a huge weakness on sourcing from other search engines is that DDG are bound to the terms of those partnerships. Or a complete severance of them. No partners, no search results.
One result of the terms in these partnerships is that DDG can't provide a search API. There was a time I thought perhaps I could write a developers search engine with DDG as the backend. Turns out you can't, and as a small search engine popular with HN types, I feel like that is a huge weakness.