The Qur'an challenged those accomplished in Arabic expression to produce an effective counterpart. Its first audience included experts in eloquence who had the motive and opportunity to respond; an ordinary poem or piece of prose would not satisfy the specific challenge.
Qur'anic inimitability involves its mode of expression, arrangement, meanings, guidance and the setting of the challenge. A counterexample must answer the claim being tested, not merely impress a reader by unrelated literary preferences.
The expertise of the original audience matters because the challenge was issued in a field of acknowledged linguistic skill.