It was through Plato's severe criticism and outright rejection of Homer's epic poetry that an authoritative rational discourse, known as logos, was established.
At which point, in a typical moment of bathos, Aristotle concludes his discussion of tragic and epic poetry with the words " Let this count as sufficient discussion" (1462a19).
Diegesis is also a form of mimesis, namely the imitation of narration, the retelling of stories that were previously told, which is indeed one plausible way of approaching the origins of epic poetry.