In the AdminCP, there is a place to edit permissions for each Super Moderator individually, by going Forums & Moderators --> Show all moderators --> Super Moderators --> Edit permissions
Before editing permissions, there should be a checkpoint to see if that user is included in the config.php file as being unalterable.
I would consider it a bug, but I'm not sure if it fits better as a suggestion for new features.
I first discovered this because I wanted to restrict my admins from being able to physically remove certain things, like posts, from the database. I found this permission in the AdminCP, but then I found that changing these permissions to "no" is pointless, because this user can just go to the same screen and give themselves unlimited moderating access.
I think that when a user is set to unalterable in the config.php file, that should also include this screen of Super Moderator permissions.
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