It can be both art and programming! Here's a paper showing how programmers work with artists to develop the classic TF2 half lambert [0]. This shader also makes use of an efficient method for representing diffuse environmental contributions using low order contributions on a spherical harmonic basis which is similar to techniques from quantum mechanics [1].
You read the paper wrong. They use a 6-face ambient cube instead of the SH9 term. I guess the SH9 term was somewhat expensive in 2007, but it's now pretty standard for representing ambient diffuse light, and has much better results than the ambient cube. For more on that, see this slide deck: https://steamcdn-a.akamaihd.net/apps/valve/2006/SIGGRAPH06_C...
But yes, understanding some physical basis for lighting is helpful even for non-photo-realistic rendering.
damn, I haven't played Team Fortress in so long lol. This is a cool thread, I'm going to have to Bookmark this so I can always come back to it for foundational inspiration and as a reference key for doing artwork.
[0] https://steamcdn-a.akamaihd.net/apps/valve/2007/NPAR07_Illus...
[1] https://cseweb.ucsd.edu/~ravir/papers/envmap/envmap.pdf