Under our help and f&q we explain how we come up with scores: gdgt.com/help/#gdgt-score. A quick tl;dr is that we use critic sources, hands on, and other factors to help determine a score. If you go to the Nexus Q page you'll see we have 14 different reviews on the page from a bunch of different sources.
Additionally, while this was pulled from the market rather quickly Google did put this up for pre-order so it was at one point up for sale.
Because sites like Engadget, CNET and the likes get products before they're released to customers, so that the reviews are up as soon as or even before consumers can actually get their hands on the products. These sites had the review up before google started shipping the nexus q to customers, and they decided not to release it yet.