Hadd al-iʿjaz is the limiting rank of rhetorical excellence at which human rivals cannot produce an adequate match. Within the book's framework it serves as the highest benchmark, not simply as another description of attractive prose.
Muʿarada means attempting a comparable expression; ʿajz means failing to do so. A merely striking metaphor or a poet's confidence in a line does not by itself establish inimitability.
The discussion of the Qur'an's inimitability appears in the context of the highest rhetorical rank, rather than as a routine claim applicable to every fasih utterance.