It’s not just the hardware. “The cloud is expensive” is usually touted by people not understanding why managed services (like Aurora RDS and OpenSearch as suggested in the article) ‘cost more than running it themselves’ by not accounting the management costs.
A database service needs management not only in hardware (I.e. replace dead drives) but also in software (I.e. monitor cluster performance, tweak system settings to fit usage pattern, manage cluster health, etc etc). These management requires time from the ops team, often in multiple roles like SysAdmin, DBA, and Ops engineers. Fact that they claim to have moved to their own hardware without being on new talents to their ops team makes it questionable as to whether or not they actually understand the cost and If they’re overworking their existing ops team.