Yes they are hired to find cracks in the system to ensure no dark seo methods can be used to boost sites rankings.. Hence hackers hired by companies are known as crackers .
He was referring to this: http://www.mattcutts.com/blog/ Many people are thinking that it is an April Fool Joke. If dark seo is hired by google, ofcourse the April Fool theory makes sense.
It is possible that Google may be doing that in order to get an insight into black hat seo techniques.
Here is what I think I think that the site is really hacked. Somebody who knows a little bit about the internals of http & some technical knowhow can intercept traffic over plain text. Thats why i got ssl set up in my site admin directories.
Cmon its illegal for someone to hack into someones site, someone like google wouldnt do something like that, and if so why only him? there are a million other websites out there aswell. Oh and Why didnt they hack his home page?