In a question on habits (I-II, Q54, art 2), he makes reference to Aristotle having "proven" that the Earth was round. His wording can make it seem like he agrees especially if you come at it presupposing the Earth to be round, but he does not state clearly is own opinion on the matter. He was simply using it to make another general point.

One also has to understand the progression of scholastics.  Before any of them would have the hubris to "teach" anything on their own, they were expected to simply provide "commentaries" on the works of other teachers, thus acquiring a familiarity with their material.  For St. Thomas, in the context of such a commentary, to reject Aristotle's opinion (vs. merely explaining or commenting on it) would have been considered extremely arrogant.

After having made this clear, let´s now try to understand the work of S. Thomas about Aristotle which say that the earth cannot move and, if the other arguments above were not available to him, he supposes that the better would be to consider that the earth is really flat!

 

He starts to say: Quidam, scilicet Pythagorici, posuerunt terram moveri circa medium mundi, ac si esset una stellarum,(the Pythagoreans, assumed that it is in motion about the middle of the world, as though it were one of the stars) ...dicunt eam revolvi circa medium cæli, idest circa axem dividentem cælum per medium,( assert that it is revolved about the "middle of the heavens," i.e., about the axis which divides the heaven through the middle) sed Philosophus ostendit quod impossibile est terram sic moveri.(but Aristotle shows that it is impossible for the earth to be thus in motion)

 

In other words: the Philosopher (Aristotle) excluded the opinions that the earth could spin: excludit opiniones eorum qui falsas opiniones circa terram habebant,

 

And also explains that all things move around the earth to the earth, so it must be stable and it can´t move in anyway: Assignat causam quietis terræ et dicit quod ex præmissis manifestum est quæ sit causa quietis ejus. Sicut enim dictum est, terra naturaliter est nata moveri ex omni parte ad medium :sicut sensibiliter apparet quod ignis naturaliter movetur a medio mundi ad extremum. Unde sequitur quod nulla particula terræ vel parva vel magna potest moveri a medio nisi per violentiam. Manifestum est quod multo impossibilis est quod tota terra moveatur a medio. (he assigns the cause of the earth's rest and he says that from the foregoing everything goes to the middle. For, as has been said, earth is naturally inclined to be borne to the middle from every direction, as our sense observations indicate — and similarly it is apparent to sense that fire is naturally moved from the middle of the world to the extreme. Hence it follows that no particle of earth, small or large, can be moved from the middle except by violence; so, it is plainly much more impossible that the entire earth be moved from the middle.)

 

Concludit propositum: quod terra sit in medio mundi quia omnia corpora gravia moventur ad medium terræ. (That the earth is in the middle of the universe and all heavy bodies are moved per se to the middle of the earth ) Et sic, ex præmissis, nihil movetur in loco ad quem naturaliter movetur, quia ibi naturaliter quiescit. Sed terra aliquando movetur ad medium mundi, (from the foregoing as follows: Nothing is moved in the place toward which it is naturally moved. But the earth is naturally moved to the middle of the world.) ut probatum est, ergo, terra nullo modo movetur. (Therefore the earth is not in motion in any way)

 

After all these commentaries, he concluded that to be stable, the earth must be flat: Necesse est terram, ad hoc quod quiescat, habere figuram latam:( that if the earth is to be at rest, it has to be flat.) nam figura sphærica facile mobilis est quia in modico tangit superficiem, sed figura lata secundum se totam tangit superficiem, et ideo est apta ad quietem. (For a spherical shape is easy to move, because so little of it is in contact with a plane; but a wide shape is totally in contact with a plane, and is consequently apt for rest and to be firm.)

Holy as he was and is, I was told I believe that even when he is wrong, he is right.

With that said (my previous post), the question is whether in the Commentaries St. Thomas was even expressing his own opinion / teaching or whether he was merely explaining (aka commenting on) Aristotle.  That's actually a debated question among Aquinas scholars, whether St. Thomas was accurately reflecting the mind of Aristotle or else injecting his own thinking into the Commentaries, with the majority holding the former.