DEVONthink has no built in file backup capabilities. A proper backup strategy is up to the individual to implement and maintain. Due to the simplicity of Apple's Time Machine, we suggest using it with local external drives. For an online backup, you can use a cloud backup service that uses a snapshot-style backup, ie. not "continuous backups". Arq is one such service.
Can't I use sync for backups?
While sync can be used to restore a database in an emergency, sync should not be your only backup. The sync data is chunked, DEVONthink-specific raw data about the databases and is only useful to DEVONthink or DEVONthink To Go. A proper backup should be application-agnostic.