A few thoughts:
1) Extrapolating from the infection rates of very specific groups (for example, evacuees) to the entire population without taking into account transmission dynamics and the time between infection and detection does not make very much sense. The authors naively multiply the infection rate among evacuees by the population of Wuhan to conclude that Wuhan must have had 178,000 infections at the end of January. By comparison, epidemiological models have estimated there were around ~20,000 infections at that time . What conclusion should we draw here? If you use sloppy, back of the napkin math to over-inflate the infection count by 10x then you can correspondingly deflate the mortality rate?
2) Speculating about the mortality rate of Covid-19 based on several gigantic assumptions ("If our surmise of six million cases is accurate, that’s a mortality rate of 0.01%") seems borderline irresponsible. Numerous researchers have been modeling this virus and have generally arrived at numbers in the range of 0.5% to 1.6% . The authors don't present any compelling reason why we should doubt those numbers.
3) Ultimately the mortality rate is not as important a number as the hospitalization rate. The authors would have you believe this virus is no worse than the flu, but this is not congruent with the number of reports coming out of places like Italy and New York saying they're about to run out of ICU beds, or China rushing to build temporary hospitals to house all of the patients that need critical care. What the mortality rate might be under ideal circumstances where every patient receives adequate medical care might be significantly different compared to a scenario where you've run out of ICU beds and have to start rationing ventilators.
Korea's mortality rate is about 1%, and that is primarily due to a relatively youthful population and the healthcare system not being saturated. I'll also note that 1% is still 10x deadlier than the flu.
It sounds like you may be interested in reading this paper: https://www.medrxiv.org/content/10.1101/2020.03.04.20031104v...
>I wouldn’t be surprised if the majority of us have already had it. There’s a decent shot this all amounts to a big ol nothing burger.
Unsupported by any evidence and complete wishful thinking. Please stop with the irresponsible speculation.
There have been a couple of population studies to try to measure asymptomatic infections. It appears that about 50%+ of people are symptomatic, so unfortunately significant percentages of people are going to need hospitalization.
This study shows the number of symptomatic patients broken out by age group on the Diamond Princess which was a very well-studied population. https://www.medrxiv.org/content/10.1101/2020.03.04.20031104v...
Sorry, in my mind I was addressing mrb directly who surely knows the relevant data already. But you're right I should have included citations for other readers.
The most detailed analysis of Chinese(!) CFR numbers I am aware of, including discussions of all the issues that mrb likes to quote to sow confusion: Riou et al. 
Cumulative number of tests performed in South Korea: See the table at ; ideally plot the "Total Tested" column logarithmically to see sustained exponential growth clearly.
Confirmed cases in South Korea over time . Plot South Korea alone to see the continuous decay (pretty much from day 1 to right now) of the exponential growth rate clearly.
Best study I’ve seen so far (Univ Bern, Switzerland) estimates a CFR (adjusted) for Hubei province of 1.6% (or 3.3% if symptomatic). Worse above 60 yrs, better below 40 yrs.
Presumably you think Korean health care is better? Now what a about other countries?