Yes, after they ran that segment the first time, they got a call from a Detroit auto engineer who debunked the tailgate down myth. He told them that with the tailgate up, a bubble of recirculating air is created in the bed when the truck is moving, and the recirculating air bubble helps direct the airflow smoothly over the back of the truck, decreasing drag. Dropping the tailgate cause the recirculating air bubble to collapse, and increases drag.
It’s been proved. Dropping the tailgate increase drag.