That saying also means something else (and imo more important): RAID doesn't protect against accidental or malicious deletion/modification. It only protects against data loss due to hardware fault.

If you delete stuff or overwrite it then RAID will dutifully duplicate/mirror/parity-check that action, but doesn't let you go back in time.

Thats the same reason why just syncing the data automatically to another target also isn't the same as a full backup.