The feature you are describing from the Mail Help is specific to Notifications.
When you receive a Notification of a new email setting that option allows you to archive instead of delete. It also changes the Swipe Left action from Delete to Archive. But it won't change the function of the Del key to Archive.
The previous query you referenced states that CTRL + CMD + A will Archive the currently selected emails and that does work. The original poster on that question thought they had at one point changed the behavior of the delete key in Apple Mail. But I can assure you that they are mistaken.
The delete key is used all over the place and you really cannot override it. It's hard coded into the operating system and Apple API's, etc. There are no options to override the delete keys behavior within anyone one particular App.
However you can assign a keyboard shortcut for Archive but it won't be using the Delete key.
System Preferences -> Keyboard -> Shortcuts -> App Shortcuts -> Hit the + add. the Mail app and Menu Title to be Archive and then define the keyboard shortcut you wish to use. Such as Ctrl+D which works. But trying to use Del will result an an error tone as it is a reserved key by the operating system.