wasted spent the last 30 minutes wondering why the Advanced Super Ninja Search Options weren't working for me.
It turns out that these options only work in the top right search box and not the three
Search Stack Overflow With Google|Bing|DuckDuckGo search boxes on the same page.
Whilst the opening paragraph does say:
To search, type in the box at the top right of every page and press Enter
The presence of the three search engine search boxes on that page do implicitly suggest the Ninja options will work there too. I do realise that if you look closely at the Google results it does say that user:419 was dropped from your search, but you have to look closely.
Perhaps some additional text would be useful to make it abundantly clear that the Ninja search options will only work with the top right search box?