It is clear that Epíkouros, in Book 28, takes up Plato's discussion of "conventionalist" vs. "naturalist" views of language.
A linguistic conventionalist thinks that languages come about only by convention. The conventionalist denies a word's intrinsic relationship to a class of objects and therefore asserts that any word can refer to a particular class of objects—and that no particular word is better per se for this purpose.
In contrast, Epíkouros recognizes that linguistic conventions originate from an intrinsic relationship between a class of objects and the primary expression naturally used (specific to race and location) to identify that class. Languages originated from natural impulses. Because of this, most words inherently apply to an originally specific concept. Therefore, Epíkouros recommends that we base our vocabulary upon these fundamental meanings.
So even though Epíkouros and Cratylus both disagree with Hermogenes regarding convention, Epíkouros and Cratylus disagree on the basis of "correct" usage (with Epíkouros basing this in automatic association of fundamental etymologies, and Cratylus in the divine origins of language).