Yes, but every 10lbs you put in that trailer is using 1lb of the truck's payload, assuming you load it perfectly to get that 10% tongue weight. For travel trailers, most of the recommendations are to have the tongue weight closer to 15%; the F150 in the video would've been at or over max payload with a 15% tongue weight and just the driver. Actually, it would've been close to its rated payload with just the trailer and the weight-distributing hitch (which is required if your trailer exceeds 5,000lbs); the driver probably would've put it over.
Their test was a pretty good worst-case scenario for what they were testing, but there's more to towing than fuel economy, and that particular F150 wasn't really enough truck for that task. Ford makes F150s where towing that load is well within the truck's performance envelope, but that one wasn't it, tow package or not.
The trick is to use your trailer capacity, you put as much as possible in the trailer instead of trying to use much payload, put it in the trailer and the down force is about 10% of what the payload amount would be ... passengers and pets excluded of course.