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He is talking about the two major competing cosmological models of his time. These include shapes of both heaven and earth.

There is the model of the spherical earth surrounded by a spherical heaven which he describes in this passage as: "heaven is like a sphere and the earth is enclosed by it and suspended in the middle of the universe." And there is the model of the flat earth with a dome shaped heaven which he describes as: "heaven like a disk above the earth covers it over on one side."

Since there wasn't anybody around then who thought that there was a flat earth surrounded by a spherical heaven, there is no reason he would be mentioning that possibility. A spherical earth in a spherical heaven was the dominant cosmological model of the classical world.  Of course, that is the model he is talking about.

And we see elsewhere in St. Augustine's writing (such as his discussion of the antipodes) that he leaves open the option of the earth being a globe.  He does not presume the earth is flat.  On the contrary, the majority of scholars think that he himself believed the earth is a sphere.

When was the globe officially accepted by the Church?  Do you have that docuмent? Who wrote it?  There should be something tangible out there if you're going to insist the Church accepted the globe.  Silent apostasy of Catholics against Scripture is no proof.  I've yet to see a Father who teaches how the globe works with the Scripture's descriptions of earth.  And it's pretty haughty to contend that the nature of God's creation is unimportant. Your argument lauds generations of pagans who pushed for the globe relentlessly, hundreds of years BC and AD. Their nonsense finally got the Church's attention and She condemned the Pythagorean/Copernican Doctrine calling it "altogether false".  Now, if the Church had actually accepted or taught earth was a globe, She would have singled the globe part out and said, "altogether condemned, except for the globe earth".  But She didn't.  The model was condemned altogether.  That means, entirely.  Add up every last Catholic who ever lived that believed earth was a globe and they don't even begin to outweigh the Fathers who echo Scripture and the liturgy. Too many Fathers expounded not only on Genesis, but Psalms, Job, Isaiah, and other biblical texts.  Interesting how the sketch of the flat earth is found in many older Catholic bibles.  You'll certainly never find a sketch of the globe in an older bible. This isn't even legitimately up for debate as we have Father's quotes, their teachings and how those teachings work with, and bring to life Scriptural texts. We also have the science to back the Fathers up.  The Fathers taught that church architecture, the liturgy, the vestments, the altar, the dome, the pillars, the Sacrifice, the candles, down to the scalloped edge on the altar cloth, together reflect, within themselves, a microcosm of the earth and seas. And, that the earth is a macrocosm of the Church. That's not some random notion of the Saints. That...is revelation.