Microsoft Azure doesn't agree with this author. They have 3 Availability Zones in their Korea Central region.

It seems like there are a large number of commercial data centres in operation in South Korea, and not all in Seoul: https://www.datacenterdynamics.com/en/news/?tag=south-korea

Kakao's incident was caused by a single building fire. It's quite a leap to suggest it is impossible to deliver a resilient service in a country the size of South Korea.