The author starts by telling us about three types of bronze vessels from China's Shang dynasty, then points out something all the surviving vessels of these types have in common: they are all inscribed with the taotie pattern. The author concludes that if a vessel is not inscribed with the taotie pattern, it must not be an authentic vessel from the Shang dynasty.

But the author is trying to derive a conditional conclusion — i.e., a conclusion about a whole set — from a subset that's not necessarily representative. Imagine saying that since hamburgers, quesadillas, and pizza are all popular foods in the US, and all contain cheese, any food that does not contain cheese must not be popular in the US. This would obviously be false, because there are many more foods than just these three types of food. We can't use these three alone as a basis to draw conclusions about food in general, and certainly not to conclude that cheese is either a necessary or a sufficient condition to make a food popular in the US.

Similarly, the author of this stimulus has only told us about three types of Shang dynasty vessels. Just because all the surviving vessels of these types (and for all we know, there were vessels of these three types that have been lost, but didn't fit the same pattern) have this taotie pattern doesn't mean that no authentic vessels from the Shang dynasty do not have the taotie pattern. There could be many other types of authentic Shang dynasty vessels that have different patterns, or no pattern at all.