The Martian is an example of "hard science fiction," or a subset of the genre of science fiction. Hard sci-fi, or hard sf, relies on scientific laws, empirical data, and the conditions of reality as human beings currently experience and understand them. This reality undergirds the fiction of the text, with the idea that any technologies projected into the future are conceived with current scientific consensus in mind. In this sense, hard sf has as much in common with "realist" fiction as it does with the broader genre of science fiction; hard sf might be seen as a fusion of realist principles with the future-oriented viewpoint typical of much sf. As Mya Nunnally puts it, hard sf "exists inside the realm of scientific possibility [emphasis added] ... [and] anything that occurs in the story is not outside the known physical laws of the universe" ("Hard Science Fiction: A Beginner's Guide to the Genre"). (Nunnally's article also points out some of the "tropes" of hard sf, including "advancements in technology," and offers a diverse array of examples combatting what she calls the typically "almost unanimously white and male" creators associated with it; she mentions The Martian, too, saying it "[makes] science cool and fun again.")
When one reads The Martian, one knows, for example, that time can only run in one direction; that if Watney dies, he cannot come back to life; and that China and the United States are important global superpowers with complex institutional support systems in place. These are the sorts of things one also assumes when reading, for example, The Catcher in the Rye (an example of mid-century American domestic realism), and similar assumptions form one's interpretive network for receiving the text. Weir doesn't really need to do inordinate amounts of "world-building" in writing The Martian, because so much of the story derives from the intellectual landscape in which we already live and work.
It's important to note, however, that hard sf can contain some elements that don't currently exist, or that might not exist in the future in the form specified in the text. The ion engines that power the Hermes, for example, don't yet exist in NASA's technological arsenal, and though their design derives from what Weir and engineers imagine an ion engine to be, it wouldn't be possible to compare the book's engines to "real" ones already existing. It's thus not strictly true to say that hard sf books like The Martian don't conceive of worlds beyond the world we're in; they do make these kinds of guesses, the kinds that can be proved incorrect over time. But these guesses are intimately bound up in the experience of the empirical reality we talk about now, thus making them somehow immediate and believable, even as they encourage the reader to employ her imagination.