The Site Editor permission is a default setting that's put in place for all Broker/Admin users on a platform, and provides additional access within the WordPress console for Broker/Admins to make edits to overall website settings through a new Customize Site page in your WordPress dashboard.
The expanded editing options for Broker/Admins through the Site Editor permission include:
Homepage porthole information updates (this is specific to websites with the Hawkeye theme only)
At this time, there is not a way to turn off the Site Editor permission for any Broker/Admin user, and it will be defaulted on for each user with the Broker/Admin permission on a platform.