Why wrong? He says that right in your quote “the nature and origin of the human race”.
And yes, he believed it happened in an instant. My point is why should I believe you and not him? You’re not claiming to be infallible on your interpretation of Genesis 1 are you?
I’m claiming that there was no reason for Augustine not to accept a straightforward reading of Genesis. He belonged to the neo-Platonist Alexandrian school, and tended to allegorize scripture because of his prior committment to Greek philosophy. But that doesn’t change the fact that Augustine believed in recent creation.