From the summary article, it looks like they have made a Fresnel lens, which can also be quite flat but works over a limited wavelength range of light.
In order to achieve broadband operation, the phase change vs wavelength of the metamaterial needs to approximate a large delay. If a large delay is not feasible to implement, this summary is overstating the importance of this result compared to prior technology.
In order to achieve broadband operation, the phase change vs wavelength of the metamaterial needs to approximate a large delay. If a large delay is not feasible to implement, this summary is overstating the importance of this result compared to prior technology.