5780 K = surface temperature of sun
63.3 MW/m^2 = thermal radiation from surface of sun from Stefan Boltzmann
46,400 = (1 AU/radius of sun)^2
1360 W/m^2 = (63.3 MW/m^2) / 46,400 = solar insolation at earth (top of atmosphere)
(1000 W/m^2 = approximate solar power flux at surface)
So a plane surface facing the sun at noon would receive about 1 kW/m^2. With a single plane mirror you could approximately double this number to 2 kW/m^2 by re-directing light that would have fallen somewhere else. Add another mirror and you are up to 3 kW. Change to a parabolic mirror and you could make this many times larger.
The question is "what it the maximum possible amount of sunlight you could focus onto a surface?" I would conclude that the maximum would occur when sunlight is focused from every possible direction so the surface "sees" the sun in every direction. Then the temperature of the surface would rise toward the temperature of the sun (radiative equilibrium). I.e. you get 46,600x the solar insolation = 63.3 MW/m^2.
The paper seems to claim 72 MW/m^2 for the focused sunlight, which would drive a blackbody absorber to a temperature HIGHER than the temperature of the sun = heat flowing from a cooler object to a warmer object!