the point is that even with 100% INefficient solar panels the pyramidal sides can be made to have an arbitrarily large area, and due to convexity of the pyramid each infinitesimal surface element of the radiating sides can emit the full hemisphere, so given any target temperature, we can design the pyramid sharp enough (same base, different height, so that heat absorbed is constant and heat emitted must equal it in steady state, then by basic thermal radiation math, the asymptotic temperature it will settle at can be made arbitrarily close to temperature of the universe, by making the pyramid sharper.)