People in the future will be richer due to whatever economic productivity growth we have. It is not true that “people” will be richer due to economic growth in its pure form, although society will be richer. There is also the problem of planetary carrying capacity to consider — that is, with increasing population comes increasing liabilities in addition to increased production.
Nonetheless, I agree with the hypothesis of the article (that a discount rate is necessary for calculation of ethical carbon tax rates). Less trivial is the question of which rate to use.