They're talking about the incoming irradiance vs. the surface reflectance. Both are related to the surface normal, but the former has to do with surface visibil

They're talking about the incoming irradiance vs. the surface reflectance. Both are related to the surface normal, but the former has to do with surface visibility, rather than reflectance. You'll notice that BDRFs don't include the Lambert cos term, since they only describe how they deal with incoming irradiance. Put simply, a surface can't bounce any light if it doesn't receive any light to begin with. It's a weird, nitpicky statement, but hey, that's what this site is about, right? > All lighting is just light bouncing off of things In real-time graphics, bounce lighting is pretty rare. Typically you cache incoming irradiance into some form offline to emulate bounce lighting. Most of our scenes are lit with analytic lighting. A key directional e.g. sunlight for most light, and then punctual sources like point and spotlights for additional lighting tweaks.