[Whitestrake]

This was my understanding too. Their Sources page[1] indicates that they run a crawler called DuckDuckBot (implying some indexing takes place), and have community maintained/crowd-sourced data as well.

[1] https://duck.co/help/results/sources

[bduerst]

Those are mostly DDG's instant answers, if I'm not mistaken, where websites use the SDK to submit their data to be crawled.

DDG doesn't have webmaster tools or crawls domains natively.