The lower endpoint marks the minimum limit of balagha. Beneath it an utterance may lose the relevant degree of suitability and cease to qualify as baligh. This is not simply a debate about correct or incorrect case endings.
Discourse may be grammatically correct while failing badly at audience sensitivity or effective presentation, and therefore not attaining the standard of baligh discourse.
The distinction runs through the opening chapters: syntactic correctness may be necessary, but rhetorical attainment is an additional matter.