I strongly disagree
The talk article says "The radiant flux for physical sources falls off at least as fast with angle as cos(?)"
"for physical sources" he is saying that _in real life_ it never happens ...
That is why I created the non-Lambertian geometry above
the surface is not a "physical source" it is constructed deliberately to isolate the cosine / projected area term in the radiance equation
and expose it for what it is - highly confusing!
What happens at 90° is not described in the radiance equation ... it is due to occlusion ... there is no radiance because you can not see the surface
The key to understanding radiance is understanding what happens when the viewing angle _approaches_ 90° ... the radiance approaches infinity
This is due to flux density: