If you read the rating, it's not so much a quality rating as it is a speed and availability rating:
Airline Flight Search Benchmark
This Gomez benchmark represents the speed, availability and consistency of a typical
flight search transaction for the nation’s largest airlines and flight aggregators. This
includes tracking the home page, departure, return flight and itinerary pages.
• The Gold standard for airline Web performance goes to Delta Air Lines, with JetBlue Airways and AirTran taking the Silver and Bronze Awards.
• Delta Air Lines had the best average ranking for 2009 with average response time at 5.65 seconds, availability at 99.50% and consistency at 5.14 seconds.
• Continental wins the Most Improved award with the largest decrease in response time from the first half of the year to the second half, 34 percent.
Compared to some other airlines, doing a straightforward flight search at dl.com is pretty fast, so I'd have to agree with this rating.