The skin layer planet is optically very thin, so it doesn't affect the OLR significantly, but (absent direct solar heating) the little bit of the radiant flux (approximatly equal to the OLR) from below that it
absorbs must be (at equilibrium) balanced by emission, which will be both
downward and upward, so the flux emitted
in either
direction is only half of what was
absorbed from below; via Kirchhoff's Law, the temperature must be smaller than the brightness temperature of the OLR (for a grey gas, Tskin ^ 4 ~ = (Te ^ 4) / 2, where Te is the effective radiating temperature for the planet, equal to the brightness temperature of the OLR — *** HOWEVER, see below ***).