There's another important caveat: this just applies to the lens element -- the piece of glass. Your 70-200 would still need a (virtual) 200 mm between the front element and the sensor.
And there'd be another problem: this lens is far thinner than a sheet of plastic, and there's no way you'd be able to keep it rigid without doing something like...well...encasing it in glass.
Lastly, even at the infrared and longer wavelengths this works at, it's horribly dark -- it only lets through 10% of the light.
There's no need to completely despair, though; the technology is quite similar in principle to the diffractive optics Canon already is using in its green stripe lenses. I imagine, with time (maybe a lot of time), all the supertelephotos will have diffractive optics. Some day we'll probably see a 400 f/2 that's as light as a 70-200.
You'll still need a second mortgage to buy one, though....