That's possible because they count both photovoltaic and thermal conversion efficiency. I'm not entirely clear on whether they want to use the heat as heat or always for electricity generation. They claim GaAs solar cells get 40% efficiency, which I'm ready to take at face value. 60% of the energy is then converted to heat. For a total of 80% conversion efficiency we'd need only 66% efficiency for the thermal-to-electricity conversion, which is well within theoretical reach, given they also claim >1000C achieved temperatures.