I'm SURE that didn't skew the results at all.
They respond to this in the study, but it's going to take a lot to convince me that Mechanical Turk is a respectable source of subjects for consumer behavior research.
The real news here is that Bing and Google are virtually tied with respect to search quality. But that's been my experience based on using DuckDuckGo (which is basically a rebranded Bing afaik).