Sometimes the most obvious explanation is the correct one. But before confirming your suspicions that there’s very little to this saying, I’d like to say that this phrase exemplifies the loftier goals of KCDFYS: Taking a second look at what we take for granted or don’t think twice about.
Anyway, yes: If you thought “shit from Shinola” meant that Shinola shoe polish (1907-1960) had a color and texture “not unlike feces,” as Wikipedia puts it, you are correct! What you may not know is that this became popular during WWII. Why? Wartime makes intelligence more important, I guess.