St. Thomas says that Sacred Doctrine is a single science, rather than a collection of sciences. It is distinguished by its formal object, which is all that has been divinely revealed.
The unity of a faculty or habit is to be gauged by its object, not indeed, in its material aspect, but as regards the precise formality under which it is an object. For example, man, ass, stone agree in the one precise formality of being colored; and color is the formal object of sight. Therefore, because Sacred Scripture considers things precisely under the formality of being divinely revealed, whatever has been divinely revealed possesses the one precise formality of the object of this science; and therefore is included under sacred doctrine as under one science. [ST I Q1 A3]
That’s not to say that sacred doctrine has nothing to say about anything else, but that it only has something to say concerning them insofar as revelation addresses or relates to it.
Some Protestants like to pretend that St. Thomas held to various Protestant distinctives that are contrary to the teaching of the Catholic Faith, and some members of that little band might try to say, on the basis of one clause above, that Aquinas held to “sola scriptura.” But when he says “because Sacred Scripture considers things precisely under the formality of being divinely revealed,” he does not mean to say that this is the sole locus of divine revelation, which ought to be clear from what immediately follows: “whatever has been divinely revealed…” It’s obvious that he doesn’t mean to limit revelation to what is contained in the Bible. Consider the preceding example, in which he identified a commonality of “man, ass, stone” in “the one precise formality of being colored; and color is the formal object of sight.” Analogously, he describes Scripture as dealing with things “precisely under the formality of being divinely revealed,” and says that whatever has been divinely revealed “possesses the one precise formality of the object of this science.” So he’s not trying to limit the scope of divine revelation to the Scripture here; rather he’s he’s trying to define the scope of the science of Sacred Doctrine as having to do with whatever has been divinely revealed, and addressing other things under that aspect.
Of course, this doesn’t mean that we can’t have systematic theologians or moral theologians or a theology of man alongside a theology of redemption or whatever; since the scope of what has been revealed is broad, and since the truth is likewise so deep, it’s reasonable to have a division of labor with regard to the science of Sacred Doctrine. We can’t all be Aquinases who are experts on practically the whole of the field!