Regardless of that interpretation of St. Thomas, St. Augustine did relate the debate about the school who thought earth (w/ firmament) was hemispherical, since the heavy parts settle at the bottom of the water (i.e. dirt is heavier or more dense than water). If that's their thinking one would certainly think the bottom of it was flat (well, that's why they say it's a hemisphere).
But, again they didn't have a concept of gravity until the Mason Newton invented it to explain cosmology, so they didn't have a notion of how someone could adhere to the bottom of a ball, for they did have absolute concept of up and down, not relative to the earth "core".
St. Ambrose, in that one passage, addresses the objection about how the world (which he called a sphere) could be suspended in the midst of the waters. His answer was that the waters spun around the spherical earth (the firmament snow globe) and this swirling motion kept the earth suspended, vs. settling to the bottom. That was the state of the debate back then, where NASA's ball floating in space occurred to absolutely no one. Of course, if the waters swirling around the "sphere" kept the entire thing suspended, he's obviously talking about the firmament enclosure, and not the ball surface as NASA would have it, because at that point the earth would be inundated by said waters.
So if St. Ambrose had to construct an argument to explain how the earth sphere didn't sink like a rock to the bottom of the universe, then he clearly also would find absurd the idea that something could stick to the "bottom" of a ball (since, again, for him, up and down were absolutes).