I think the author is trying to say that there is a concept of knowledge that is inherent to words "perfect forgery". That either someone knows the origin of an object and therefore it is not a perfect forgery, or all such knowledge has been erased and now there is no way to say that an object is not authentic.

I think this is something of a straw man argument. There is an entirely satisfactory functional definition for "perfect forgery" which is that an object is a perfect forgery if it cannot be distinguished from an authentic object by someone with no prior knowledge of the object's origin. This is what I believe common intuition would determine the word "forgery" to mean. An object does not fail to be a perfect forgery because its creator knows it is not real. It is a perfect forgery if it can fool everyone else into believing it is real.

Making zero knowledge of authenticity a prerequisite for a "perfect forgery" is to neglect that there are multiple independent minds in the world with their own viewpoints. I can know "that is a perfect forgery" if I know it will fool anyone without my private knowledge. Such knowledge is not incompatible with the object being a perfect forgery, because the most sensible definition of "perfect forgery" is that it would fool anyone else, not that it has already fooled everyone including me.