An alternative wording of that statement could be "The average of the [already averaged] Yelp ratings of the restaurants listed on this page is over 4 stars."

I don't think that they're claiming that every restaurant has a 4-star or above rating. The claim is that the cumulative average of the 60-some restaurants on that page is over 4 stars. It's possible that they have 46 5-star restaurants, and 14 1-star locations.