Ah, so when the Samba client is installed as well or was previously, or an SMB mount was added via dietpi-drive_manager. Strange that a Samba server config file is shipped with a package shared between server and client, instead of with the server only.
However, the file is actually coming from /usr/share/samba/smb.conf, so we can check whether /etc/samba/smb.conf is the same, if it exists, to decide whether to install our config.
EDIT: Fixed with: v9.1 · MichaIng/DietPi@8139155 · GitHub