I'm quite comfortable with calling BS on that
The "theory" is complete gibberish "The solution .. to create a vortex out in front of the car", not even worth independent testing IMHO (and some folks added roof racks to hold their creations).
I don't know WHAT the therory is supposed to be, if it is supposed to make thrust, or maybe just seeing it behind you scares you into wanging open the throttle and reducing pumping losses.
There *might* be some cases where a spoiler can help CDA as a band-aid for crummy aero body, but do not expect anything like "...have all seen improved mileage from at least 10-mpg; but most have at least doubled their mileage, and a few of us have achieved 3-x our normal mileage on a limited basis. " Rest assured that is 100% bullcrap.
edit: more overwhelming evidence of solid bullcrap:
"We have a wing on a 78 Lincoln Town Car with a 400 CID engine that is being driven 109 miles round trip daily to work here at Firestone. We tweaked it and are getting 56 mpg
out of it at present.The 78 Lincoln Town Car averaged 17.8 mpg before the wing was placed on it."