Somebody has already explained it in the comments in the article: "The light coming in the from sun is pretty directional, and a parabolic setup will focus it

Somebody has already explained it in the comments in the article: "The light coming in the from sun is pretty directional, and a parabolic setup will focus it down to a handy point. The photons being emitted by the the chip are more diffuse and so most will be reflected back down to be re-captured."