the ckeditor is already secured, it can only be accessed if you are logged in as an administrator.
The entire power store installation is in a "Public" File space.
The admin folder itself does not change the access requirements of the files inside of it. Each file in the admin folder has code on it to restrict access to only administrative users.
accessing the admin menu php file directly will not work, it will generate a PHP error because helper files will not have been defined.
If you upgraded from PS4.x to PS4.5 you can delete the old HTMLEditor folder, this folder is not included in the PS4.5 fresh install.