It would be a self-discipline problem if the affected people were showing less self-discipline than others. The author is saying that a rich person (that does n

It would be a self-discipline problem if the affected people were showing less self-discipline than others. The author is saying that a rich person (that does not currently have the self-discipline problem) put into this situation would behave similarly. Thus the core issue is not with the poor people having less self-discipline, but instead with how poverty affects our self-discipline.