According to the OED online, rhetoric is "The art of using language effectively so as to persuade or influence others, esp. the exploitation of figures of speech and other compositional techniques to this end; the study of principles and rules to be followed by a speaker or writer striving for eloquence, esp. as formulated by ancient Greek and Roman writers.
So if one can see the words and their concoctions how is this not truth and knowledge? HOw can we understand something, but there later not be a universal understanding? How does a different interoperation make their understanding meaningless or less correct than someone else? And if there is a universal understanding than why is everyone not thinking along the same lines, or the line that Locke is speaking of.