What does the expansion symbol of From the Vault: Transform mean?
The quote is merely referring to the fact the expansion symbols colours on the front are inverted on the back. The standard is for expansion symbols to be the same regardless of what side they are printed on, so this difference in colour makes the set symbol unique. 

I suspect that the symbol is supposed to be the back of a werewolf in front of a moon. Though the expansion only contains one werewolf, werewolves are a core part of the transform mechanic and featured heavily in the mechanics debut during Innistrad.

Front (Orange background, Black lines):


Back (Black background, Orange lines):