This appears to have some unintuitive consequences. When I searched for "porn" in a cookie-less session just now, there were still porn results, but no well-known sites (at least I didn't recognize the names). Searching for literally "pornhub", the first result is "porhub.com" without the "n".

Seems like the "SafeSearch" filter is based on a list of "adult domains" instead of the indexed content at the URL.