That's Augustine's word. The bible is supposed to be God's word...
Surely to a Christian one must trump the other.
So your authority that God was being poetic is Augustine.
I'm pretty sure Christianity gets to define its own scriptural hermeneutics. To see something as the word of God (for Christians this does not mean the same thing as when a Muslim is calling the Quran the word of God, the Bible is not divine dictation) does not entail that it is read as a scientific account or even as history in the case of Genesis. A myth can be the word of God insofar as the point of the myth is considered to say something true.
And no, the authority is not simply Augustine, it is church tradition. Allegorical readings goes much further back than Augustine. The Bible never existed in a vacuum, read only in light of itself, as some modern Christians seems to believe. The Bible was first and foremost read in the light of Christ, Christ is the word of God and the absolute revelation in the Christian faith.
So no, Augustine does not trump God, but scripture is scripture insofar it is read in the light of Christ by the church in light of its traditions (that is theology, worship and liturgy). And for them, Genesis was not simply a descriptive account of origins.
Plus I've read many sources saying it doesn't appear to be Hebrew poetry from how it is written.
Correct, but ultimately irrelevant for Christian theology.
First of all, there is not one account, there are clearly two different accounts which are very different from each other and they have different origins. Genesis one was probably a polemic against contemporary creation accounts where natural entities such as the stars, sun, moon and so forth were divine entities themselves. Genesis 1 opposes this by talking about them as created. It has also been seen as a polemic against the idea that the creator God created by violence, that is imposing his power upon some pre-existing matter or in some cases a beast, in Genesis 1 God simply speaks things into existence (although this is disputed by some experts, since the verb
bara does not really entail creation from nothing as the word literally means "to form"). That is all historically interesting, but not really relevant to Christian theology, at least as it is classically understood.