The point is a parody. The hidden and non-straw argument is this: it's not that the people in Atlas Shrugged don't understand that manual labor will be needed -- it's that they don't understand that, if they are the only ones left and they have to perform their own manual labor, they are essentially denying their ability to be the technological and intellectual titans they once were. After all, if your tilling earth, you're not developing new Rearden Metals or guiding the decisions of Taggart Transcontinental or Waytt Oil.
In short, Atlas Shrugged fails to account for the fact that the elite separating themselves from society in the novel's proposed fashion also prevents them from utilizing the benefits of the society they left behind. In shorter, being an elite intellectual powerhouse means nothing without the efforts of all of the people below you. In even shorter, society is interdependent, and Atlas Shrugged goes a long way to pretend otherwise. In shortest, the rich need the poor.
Rand's philosophical notion (that state intervention in the private sector is always wrong) only works when the rich adequately support the poor just as the poor support the rich. When the rich act like dissed middle schoolers and try to leave and make their own club, they're shooting themselves in the foot.