Thats a very important question, as without an answer it seems like the original comment is just excellent Public Relations. *Did not mean to imply that it is PR :)

I asked a member of the webspam team to prototype the extension after reading this Hacker News comment on January 1st: http://news.ycombinator.com/item?id=2058408

We've absolutely done similar things in the past, but that comment was the spark behind this most recent Chrome extension. I hope that's specific enough. :)

I think the fact that Matt frequently comes in here to discuss Google's search results shows that it is not just PR.

Unless you want to categorize everything Matt says publicly (whether here, on his blog or on twitter) as PR...

Market validation. :)