You can study only the cases you find, and other scientists have estimated there could be 10 times as many people infected as are ending up in the official statistics. That means the overall death rate is likely to be lower than the one reported in this study.
The study suggests that the downward trend in the overall epidemic curve could mean that "isolation of whole cities, broadcast of critical information (e.g., promoting hand washing, mask wearing, and care seeking) with high frequency through multiple channels, and mobilization of a multi-sector rapid response teams is helping to curb the epidemic".
However, days later, a woman who had been on the ship tested positive after arriving in Malaysia. Huge efforts are now being made to track down the passengers, who have moved on to many different countries including Malaysia and Thailand, but also further afield to the US and Canada.