When St. Augustine does write about a sphere, he's referring to the entire shape of the world that includes the enclosing firmament.  One of these days, when I have time, I'll write it up.  In the same passages where he's discussing the sphere, he's also going through theories about how it may be a hemisphere, which would be located at the bottom center of the universe, indicating that it would be acceptable since bottom center is still center.  NO FATHER believed that the earth was a ball.  When they speak of spheres they're including the firmament as part of the shape.  This is easily demonstrated.  They all believed that the firmament enclosed the world and kept literal / physical water (not "space") off the surface of the earth.

Having skimmed this study, the guy cites the actual passage but misses its implications.  In fact, he even talks about how St. Augustine defined the world as consisting of two parts, the earth that we walk on and then also the sky above (up until the firmament).