Looks like they do mention that elsewhere: https://www.fastmail.com/features/reliability/

> Fastmail has some of the best uptime in the business, plus a comprehensive multi data center backup system. It starts with real-time replication to geographically dispersed data centers, with additional daily backups and checksummed copies of everything. Redundant mirrors allow us to failover a server or even entire rack in the case of hardware failure, keeping your mail running.