It would be wrong to think that authorial intent is irrelevant for Augustine. Indeed, he thinks that it is important, where recoverable. But he does not view it as the absolute key to interpreting Scripture. He thinks that where authorial intent is in doubt (as it must sometimes be), a variety of interpretations – constrained by the rest of Scripture – can be celebrated.
In this quote from the Confessions, he indicates that discerning the Bible as divine truth is more essential than discerning the intentions of the human author (Confessions, book 12, chapter 24):
But which of us, amid so many truths which occur to inquirers in these words, understood as they are in different ways, shall so discover that one interpretation as to confidently say ‘that Moses thought this,’ and ‘that in that narrative he wished this to be understood,’ as confidently as he says ‘that this is true,’ whether he thought this thing or the other?
And in this quote from On Christian Doctrine, he considers that varied interpretations of Scripture (constrained by doctrine and the rest of Scripture) might even possibly have been envisaged by the human author – and would certainly have been positively envisaged by God (On Christian Doctrine, book 3, chapter 27):
For the author perhaps saw that this very [variety of] meaning lay in the words which we are trying to interpret; and assuredly the Holy Spirit, who through him spake these words, foresaw that this interpretation would occur to the reader, nay, made provision that it should occur to him, seeing that it too is founded on truth. For what more liberal and more fruitful provision could God have made in regard to the Sacred Scriptures than that the same words might be understood in several senses, all of which are sanctioned by the concurring testimony of other passages equally divine?