They simply proved that the tailgate up actually does improve mileage over tailgate down. It has nothing to do with a cover on the bed.
As mentioned, with the tailgate up, the air that passes around the sides of the cab and the top of the cab will strike the tailgate and swirl back causing a "bubble" of air type effect. This causes higher speed air that travels over the cab to glide across the "bubble", and over the back of the tailgate. This reduces wind drag, and has the best MPG over tailgate down, which creates drag on the back of the cab/bed area.
With a bed cover (Be it "cloth" or a hard cover) the air that travels over the top of the cab now swirls behind the small section of the cab, and as it passes over the edge of the tailgate, will swirl there as well. This in essence would create two places of "drag" (Albeit small areas) where the open bed would only have one. (The area behind the tailgate, on the outside)
Theoretically then, a bed cover would actually increase drag but by a very small amount. It will be interesting to see if the OP gets long-term fuel economy gains. No way to truly tell over 10 miles. Heck I could change my driving by a small amount and gain 1 MPG if I wanted. lol. Keep us updated OP!