This is what Wikipedia has to say about happenby's notable authority on St. Augustine:
It is quite possible that any claims that he made about St. Augustine believing the earth is flat were part of his satirical writings. I would not expect happenby to be able to tell.
I did a bit of research into Leo Ferrari and his claim that St. Augustine believed the earth was flat.
Leo Ferrari does seem to have written a scholarly, not satirical, paper in which he challenged the majority opinion among historians that St. Augustine believed the world is a globe. While I have not read Ferrari's paper, I found a scholarly critique of it which gave counter-arguments for all of Ferrari's points and specifically mentions the passage I quoted in the OP.
This author states that it "is completely ignored by Ferrari even though it can be read as an unequivocal endorsement of the spherical model"
...this passage includes an unambiguous description of the earth (covered by water)as a globular mass (globosa moles). This creates a considerable obstacle to anyattempt to impute to Augustine a flat-earth cosmology.
C.P.E. Nothaft, "Augustine and the Shape of the Earth: A Critique of Leo Ferrari," Augustinian Studies 42:1 (2011) 33-48