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.