A listener who knows the same assigned meaning of several words can understand that meaning through each of them. At the level of established assignment, each word stands in a conventional relation to its sense; one is not simply assigned while the other is only half-assigned.
For example, someone who knows both “asad” and “layth” as words for a lion can recover the animal through either. Greater familiarity or frequency can affect speed of recognition, but that is not identical to whether the word has a conventional meaning.
The argument therefore concludes that replacing one direct synonym with another does not, merely by virtue of convention, provide the distinctive variation of indicated clarity sought in bayān.