Yup, that site seems to be basically redirector to Google redirector [1].

You can get the very same effect like DuckDuckGo keyword search but with "just the last redirect" with vanilla browser bookmark [2] with `%s` interpolation:

    https://www.google.com/search?btnI&q=%s%20site%3Adeveloper.mozilla.org

[1] https://github.com/lazd/mdn.io [2] https://www-archive.mozilla.org/docs/end-user/keywords.html - old page about feature in Firefox (currently the same works in Google Chrome as well). Amusing that the this `btnI` search is in the second example there.