I always recommend to keep at least a Moodle-only admin, not tied to Joomdle.
This is so you can always access Moodle as an admin, even if there's some issue with the Joomdle connection.
> Does this mean that I need to create in Joomla a new user that woul have Administrator rights in Joomla? In other words, Joomla Admin cannot be Moodle Admin?
If you want to do this, you can do it, but I removed the option to do it automatically, to prevent users from getting locked out of their Moodle admin account if there are issues with Joomdle.
So, if you want to do this for an admin (please keep another one moodle only) you can do it in Moodle: edit the user and set the auth method to Joomdle. That will sync the accounts (assuming that you have a user with the same username in Joomla)