Is it yet another chrome engine client?

They’re using whatever’s built into the OS, because they don’t want to be just another Chromium fork

They’re using whatever’s built into the OS

looks at Edge

Maybe I'm misunderstanding you, but isn't that just more Chromium?

You’re not misunderstanding at all, and you’re exactly right:

arstechnica.com/…/duckduckgo-offers-its-privacy-c…

Like its Mac browser, DuckDuckGo (DDG) uses “the underlying operating system rendering API” rather than its own forked browser code. That’s “a Windows WebView2 call that utilizes the Blink rendering engine underneath,” according to DuckDuckGo’s blog post. Fittingly, the browser reports itself as Microsoft Edge at most header-scanning sites.