Maybe it's just a semantic argument because I am a search engine developer, but I wouldn't really call DDG a "search engine". It uses Bing to power results. I would describe it as a "search interface" that gives you advanced search related tools in addition to standard search, some stratified results, and then it takes a privacy angle to differentiate itself. As such, I think the move to browser safety is a natural extension of what they do.

That said, how the heck do they make money? I only very rarely see a single ad, and there isn't much money in NOT tracking people. It's very hard to make money on search if you aren't Google (I would know, I have tried several times).