Basically, the moment it was offered for sale, the product was purchased practically sight unseen (altho the article mentions that he in effect pre-released portions of the text to his newsletter subscribers).
Given this, the book could have been low quality and it wouldn't have impacted this sales curve. In which case, the OP could have spent 5 minutes, 5 hours, 5 days, or 5 months writing the book and the sales curve would look similar.
What I gleaned from this article was not that it took substantial effort to write a book to evoke this sort of sales curve, but rather there was a process of building expectation and an audience which would result in substantial sales for a book, regardless of its level of quality. The level of quality became assumed and did not require prior evidence. Thus, labor / effort in writing was not proportional to sales.
The actual time vs. value comparison is irrelevant, though. Regardless of the time and effort it took to make, the 26k figure is pretty remarkable. We would not be having this discussion if he had made 1k-2k each day over the course of the past few weeks.
Moreover, the true quality of and time invested in the book doesn't change the fact that those who bought in the first day likely did so out of ignorance and without knowledge of the quality.
Given the amount of work put in, is it a truly huge payday for Nathan? Not really. Is it still a massive one-day sale figure? Yes.