The economist isn't really concerned with whether consumption is good or bad, but takes it as a given that people want to consume. When the hypothetical protagonist in the story buys himself a yacht, he's satisfying his own desires by redirecting the productive efforts of many Kongolese towards his own end. You can layer a value system on top of this, and argue either that it's immoral for one person to be able to direct the energies of thousands of other people for his personal efforts, or argue that it's moral because he previously delivered more value to the Kongolese through his grain speculation than he is consuming through the building of his yacht. But those moral judgments are separate from the fact that under the monetary system in this thought experiment, he has both the ability and incentive to direct resources in this manner.