Austin's proposition is that the statement, "I promise," is a promise not a proposition.
But that does not make it a description of a state of affairs [or a picture of reality per Wittgensteinian]. A description would be, "You promised," and that is clearly not a promise.
Austin doesn't say performatives aren't propositions. Note that there are still truth-evaluable performatives, and that as Austin and others continued down the il/perlocutionary rabbit hole, they came to regard all language as essentially performative.
Don't get me wrong though, I see the point you want to make; but it misses the mark in statements like Rimbaud's "Je est un autre." Writers, poets especially, do this a lot, pushing performativity to some limit where the form accomplishes what the meaning merely asserts.
Which totally reminds me of a line from Marshall McLuhan:
Just before an airplane breaks the sound barrier, sound waves become visible on the wings of the plane. The sudden visibility of sound just as sound ends is an apt instance of that great pattern of being that reveals new and opposite forms just as the earlier forms reach their peak performance.
