I’m inclined to believe them for two reasons – Firstly, the vast majority of their revenue is AdSense and AdWords based. In short, they already have a lucrative cash cow and sacrificing the integrity of search results for a marginal short-term revenue increase is senseless. People know, and Google definitely knows, that the strength of a Google search is being able to deliver the results you want to see, rather than sprinkling irrelevance all over the place.
Secondly, the writer Jeffrey Katz, runs a price-comparison website (many such sites DO adjust search results based on paid-rankings, oh the irony). Based on the somewhat emotional article written by him, I sense somebody has been jilted by a recent algorithm update.
“Let me be very clear: our unpaid, natural search results are never influenced by payment. Our algorithms rank results based only on what the most relevant answers are for users — which might be a direct answer or a competitor’s website. Our ads and commercial experiences are clearly labeled and distinct from the unpaid results, and we recently announced new improvements to labeling of shopping results. This is in contrast to most comparison shopping sites, which receive payment from merchants but often don’t clearly label search results as being influenced by payment.”