Explanation to conclusion

Hi, the argument proceeds by telling us that there is a general principle, but the general principle is disproven by an example. The author then argues that since the proponents of the principle must be aware of the counterexample, the proponents must therefore have some other motive for still supporting the principle. In this case, the idea that the economists have an ulterior motive is the conclusion, that's what the rest of the argument is structured to prove. The statement in question, the disproving of the general principle, serves to indirectly support that conclusion.