DDG absolutely is a search engine; It’s a system (engine) that lets you search. From Wikipedia[0]:
> A web search engine or Internet search engine is a software system that is designed to carry out web search (Internet search), which means to search the World Wide Web in a systematic way for particular information specified in a textual web search query.
Just because they don’t do the crawling like Google and Bing doesn’t mean they aren’t a search engine.