Mirrors merely reflect light. This device actually captures it.
You can concentrate a helluva lot of light with mirrors. Ask Archemides. There was actually a French solar plant where the U.S. tested components for resistance to nuclear blast effects, “thermal shock”. They rigged the focal point with a fast shutter and than, wham, opened it up for a couple of seconds to see how the unit under test survived the blast.
Another post pointed out the this “draws in light from space”. Basically a graviational lens is just a lens, albeit lossless. Reflective/refractive optics don’t get me excited, even if it’s refraction based on < hushed and awed tones> General Relativity.