Yes, this is pretty much what I thought.
Consequently I'm stuck with trying to get the code to work that you were helping me with on the other thread (Trying to rotate icon differently for each unique value in UniqueValueRenderer ). I'm frustrated because I am so close to getting this working. I can see that the icon does the rotation I want, but I can't figure out how to get the code to say "okay, we've done this unique value, now move on to the next one, rotate that one differently, and don't obliterate what we just calculated for this unique value and replace it with the rotation for the next one".
If I can't use ArcPy to directly use existing functionality to Vary Symbol By Rotation, this is the only option left to me.