https://docs.opsmanager.mongodb.com/current/reference/configuration/#mms.backupCentralUrl

1. It's defined as "The hostname and port of the Backup Daemon." which is not correct, it's the hostname and port of the Backup HTTP service
2. It says "You must set mms.backupCentralUrl, even if you are only using Ops Manager Monitoring and not Ops Manager Backup.". This is in contradiction to the notes in the actual file, which says

   # Note that changing the server's backup port requires updating both the mms.backupCentralUrl
   # here as well as the BACKUP_BASE_PORT property in <install_dir>/conf/mms.conf. Default
   # port is 8081.
   # Required only if using Backup
   # E.g., http://mms.acmewidgets.com:8081
   

   (I think the docs are correct in this regard, and you do need to set it)