Point gotten.

The mortality rate that the national health commission of china, the article, you and me are referring to is CFR. (x/diagnosed patient) Or rather, in your example, x/hypothesized population with the flu. "Mortality rate" by definition is the total number of deaths from a particular cause in a given time period divided by the number of people alive within the population.

The problem with the article then becomes the assumption that only 1 of 50 flu cases have been diagnosed. At worst, 20% of population is down with the flu each season, that is 66m. On the webpage 16m had medical visits (14-21m flu medical visits on another source.) Unless 14m are somehow turned away for being asymptomatic and unrecorded, "We probably find out about only 1 case out of 50." has no basis. Going by the stats on the hyperlink, 34k/16.5m is about 0.2% of diagnosed patients. 16.5m to undiagnosed 66m isn't anyway near 50 times.

This completely disregards countless other factors.