Google has provided this for years with the "site:" feature (e.g. search for "lists site:python.org"). In addition, Google uses its own results, since as the article correctly mentions "most site’s internal search engines suck".
Which is something completely different, and is also something present in DuckDuckGo.
Amazon is probably always going to be the most relevant search engine for their site, but the current choice for python searches may not be.
An example similar to python but that I have first hand experience with is Perl's CPAN searches. There are two search engines for CPAN, http://search.cpan.org and http://metacpan.org. DDG's !cpan originally defaulted to the first one, but at the time it's results were not as accurate or informative as the second one. Upon appeal the DDG people switched the default to http://metacpan.org providing everyone who used the !cpan results with what they felt was a better result.