Mallory uses the following method: "If I can not distinguish between wheat and chaff, I have to "ask" the one, who can"
Bob spreads the information that he did not receive a valid package for a serial number.
So, the problem is the following: Mallory gains knowledge by the withholding of information. Rivest had the idea that it is an advantage that Mallory can insert new packages without disturbing the communication. But the problem is that the feature always/often (?) leads to the possibility to withhold information. And that is a weakness of the concept.
So we should find a solution, which makes the withholding of information nearly impossible.