"Listen to Me, O Jacob, Israel, whom I have called: I am He — I am the first, and I am the last as well" (Isaiah 48:12). God speaks with the full weight of eternity — before everything, after everything, present through everything. And the proof of this claim is not philosophical. It is historical: "You have seen what I did to Egypt, and how I carried you on eagles' wings and brought you to myself" (Exodus 19:4). I carried you. You were there. You saw.
The rainbow enters at the end because it was God's first public covenant — the promise made to all of creation after the flood that the world would not be destroyed again (Genesis 9:13-15). Rabbi Abbahu said: in the future, everyone will come to hear God's voice — but Israel alone will say "we already know that voice." They heard it at Sinai. They heard it at the sea. The rainbow was God's promise to the whole world; Sinai was His promise to Israel; the two promises are not in competition. The rainbow covenant holds. The Sinai covenant is layered on top of it, more particular, more demanding, more intimate.
The text of Aggadat Bereshit ends where Genesis ends — with Jacob's descendants in Egypt, with the covenant still intact, with the promise still open. The flood came and went. The rainbow held. Egypt came and would go. The covenant would hold. Every generation that stood between the rainbow and the Messiah was held inside the same arch: first and last, beginning and end, the God who carries you on eagles' wings and has not yet put you down.