More recently, the US Air Mobility Command tried flying one C-17 Globemaster III some 3-6,000 ft (900-1800m) back from another, "surfing" the vortices left in the lead plane's wake – much like ducks flying in formation – and found there were double-digit fuel savings to be gained.

But Texas startup Aerolane says the savings will be much more substantial with purpose-built autonomous cargo gliders connected to the lead plane with a simple tow rope. With no propulsion systems, you save all the weight of engines, motors, fuel or batteries. There'll be no cabin for a pilot, just space for cargo and the autonomous flight control systems that'll run them.

It said "cheaper" not "energy efficient".

Wings are easy, jet engines are hard.

Besides, if you can do it with an electrical locomotive on the ground, the energy conversion to electricity of a power plant should be better than the energy conversion of a jet engine from fuel to movement.

So imo, cheaper seems plausible, energy efficient is a maybe.