The Super Admin User is used by JOIN on behalf of the user in order to access the calendar read/write rights governed by the app token. This, however, does not give JOIN access to any other rights than what is governed by the app token, nor does it give or require JOIN to have the Super Admin password.
A Super Admin will set up the Google service then create the app token with rights to the calendar which is copied to JOIN. The App token will only have the ability to listen, read, send, write, and update calendars. Again, JOIN is not capable of logging in as SA and will never have the password.