I've had a second read and I think I see the contradiction. The article says it would be fine to tax GDP (and ultimately taxing all production should be equivalent to taxing all consumption). If I have x dollars of income, I can buy x dollars of beer, or I can buy a factory that will produce x NPV dollars of beer in the future. But the GDP in those cases would be different, wouldn't it? So in a tax-all-GDP regime I should be taxed on the x dollars of income now and, in the investment case, also on my investment income (or equivalently on my owned asset).