It may well be overstated for the reason given, but equally you cannot declare that the mortality rate is '0.4%', citing the cruise ship as the only source of sample data - it is an incredibly small sample on which to make that assertion, especially as only 10 of the 700 cases has the patient fully recovered - with 4 deaths and 36 still critical/serious . The mortality rates are not fully understood and there are wide variations. In China it is now reported to be 3.5%, in Iran 11% but in Korea only 1%. There will be unreported cases, but not all that many given how tightly controlled the outbreaks are. Point is, we don't know, and at this stage I think the alarm is justified until we do. I am currently working for a Chinese company, and I've been having video conference calls with Chinese colleagues for the last month and let me tell you, the situation there was much worse than we were ever told.
This goes into detail regarding mortality rates for COVID-19
Coronavirus Mortality Rate (COVID-19) - Worldometer