Hi.
Yes, what you guessed is correct: Joomla and Moodle admins will show a disabled checkbox, so you cannot migrate them easily.
This is intentional, to prevent admins from accidentally syncing all admin accounts.
We recommend to always keep an independent Moodle admin account, not tied to Joomdle, so that you can always log in to your Moodle site, even if there's some issue with Joomdle: if connection breaks, Moodle users with auth=joomdle won't be able to log in.
As you say, you can go around this restriction, either doing it manually, or removing admin status, syncing, and then adding admin status again.
Just remember to keep at least an independent Moodle admin!
