As with all parabolic mirrors, there is a set focal point at which all of the light rays converge. The heat reflected off a flat sheet of glass is one thing. The heat reflected off a curved and focused sheet of glass is something else entirely. In order to adjust the focal length, the curve of the mirror would have to be adjustable. That would require very complex engineering and a setup involving lots of tiny little mirrors as seen in the video.

The irritating thing is that all the mirrors have to be adjustable. Perhaps a series of radial articulated arms on which the mirrors would be mounted. That would be some heavy and intricate machinery. The other limiting factor in terms of range is atmospheric haze. Range would be far greater in an arid desert than between mountain tops over a rainforest.

Still it is technology with application: like what greenies are experimenting with. A giant mirror to focus solar energy on a large container of water, generating steam for a turbine generator.