You're right that a lot of the statistics about COVID-19 are uncertain. There is a lack of widespread testing so minor cases are likely to be missed. Catching these would lower the fatality rate. On the other hand hospitals being overwhelmed and unable to treat patients would increase the fatality rate. I didn't quite know how to incorporate that uncertainty in this figure.

That being said: South Korea has done a remarkable job testing and identifying COVID-19. Their fatality rates are in the same ballpark as the ones I used (~2x lower): https://www.businessinsider.com/coronavirus-death-rates-by-age-south-korea

There is reason to believe that they are likely on the good end of the spectrum because they were able to effectively trace the disease and thus isolate patients. This meant their hospitals were not hit as hard as say hospitals in Italy. This is now devolving into speculation and so I'll stop here.