South Korea already has huge experience with SARS and MERS. Most other countries don't, so "true fatality rate" would be higher. The demographics(age distribution) is also different in other countries.
The response to MERS was a total shitshow (no laws against breaking quarantine, quarantined patients playing golf, flying to China, etc.) and we were lucky the virus didn't spread too easily. But they seem to have learned from that and are handling things really well this time: https://edition.cnn.com/2020/03/02/asia/coronavirus-drive-th...