They were being a bit imprecise with the terms. The important part is that refractive index is dependent on wavelength and so you get dispersion, which increases with the distance between surfaces. But in a regular lens, the distance is necessary in order to achieve overall deflection of the light beam's initial angle, which results in focusing. A flat piece of glass deflects light internally, but upon exiting, ever beam is parallel to how it entered, resulting in no focusing.

Being able to directly deflect at a flat, thin plane eliminates the need to consider dispersion. Well, as long as you can deflect all wavelengths at the same angle. They talk about demonstrating 3 wavelengths and seeking to extend this to more.