That isn't my premise. My premise is that the diameter of the rim and the circumference of the body of the vessel are two separate circles which the author of the text describes. It possible that the author was giving us the externa and internal measurements of the rim but I don't think so because he already gives us the the information about the thickness of the vessel.
But because its real important to the author that we are aware of the shape of the vessel in that he gives us two shapes to compare it too (a cup and a lilly blossom) its real important to the author. Additionally, by giving us the diameter of the rim and the circumference of the body it gives us the complete information needed to draw a close approximation of the vessel. If the author only gave us the measurement of the rim our ability to draw the shape of the entire vessel would be limited.
But the point remains: there are different things being measured. Even so, even if he won't accept this, the point I made about the rules of recording measurements and significant digits is still valid.