If you make money "squarely" as described in I-IV, then you have created some value. Cashing out in section V is simply consuming that value.
All we can say (given the assumptions in I-IV) is that the person was a net positive (or zero) for the Congolese economy.
The technical mistake that the author makes is not realizing that adding value to the Congolese economy means increasing the total amount of physical wealth in the economy. When the person cashes out their yacht, they are simply exchanging cash for the physical wealth they helped create.
I don't think you can logically say that I-IV are "libertarian twaddle" and that V "brings it home", because V is predicated on I-IV being true. If you're interested in the theoretical basis behind "econ 101" I suggest you look into general equilibrium theory.
It would be easy to take this line of reasoning into dangerous territory and suppose that earning money is equivalent to being morally virtuous, but this neglects externalities and the sometimes fundamental difficulty of accurately pricing certain kinds of goods (subjective wellbeing, community, a sense of meaning, etc.). Money is, at best, a very rough proxy measure for human value.
[1] http://www.meltingasphalt.com/wealth-the-toxic-byproduct/