Odes follow characteristic poetic conventions; sermons suit public exhortation; letters carry messages and arguments; and dialogue works through conversational exchange.
Each form can be eloquent, but each has familiar expectations. The Qur’an’s combination of direct address, verse endings, questions, responses, and shifts of subject cannot be fully explained by assigning it to just one of these categories.
Literary conclusion: the argument from originality concerns not only beautiful words but the overall mode of composition.