[cbolton]

I think this what-if has serious issues in its argumentation (though I agree with the conclusion):

The argument about reversibility is kind of a straw man: it answers "why can't you concentrate all light on a single point" while the real question would be "why can't you concentrate light on a smaller surface" (which you can do actually).

Similarly for the conservation of étendue: maybe you can't "swoosh the light rays closer together" but that also doesn't say you can't concentrate beams on a small surface, which might be sufficient to start a fire.

So really it all comes down to the thermodynamic argument, which has its own problem: it only works if you assume that moonlight has the same temperature as the moon. There's nothing in the article that mentions or justifies this assumption. And obviously a mirror can reflect light that is much warmer than itself, so you definitely have to explain why that's not the case with the Moon (e.g. its albedo and heat dissipation are too low).

(However I love the drawing of the encircling sun, it's great to make the point that no matter how much light you concentrate, it won't heat the body warmer than the light's temperature).