AFAIK, it's more than just yelp employees that don't admit knowing about such practices, ex-yelp employees also don't admit knowing either. So far evidence points to yelp extortion being the work of independent scammers.

However, hasn't Google reviews pretty well made Yelp irrelevant at this point? And while there's lots to be concerned about Google's business model, I'm not aware of any potential for conflict-of-interest in Google reviews as with Yelp.