Why is overriding NSViewController methods in an extension allowed?

Question

TL;DR: Why can you override NSViewController methods in a subclass using an extension when the Swift Programming Guide implies that extensions cannot be used to override functions?

In the Swift Programming Guide - Extensions section, it states:

Extensions can add new functionality to a type, but they cannot override existing functionality.

However, I routinely override NSViewController in an extension on my subclass. Example:

class ViewController : NSViewController {}

extension ViewController {
  
  // This is legal and does not generate a compiler error. 
  // The method viewDidLoad() is declared in NSViewController itself.
  override func viewDidLoad() {
    super.viewDidLoad()
  }

  // This is legal and also does not generate a compiler error.
  // The method addChild() is declared in an extension on NSViewController.
  //
  override func addChild(_ childViewController: NSViewController) {
    super.addChild(childViewController)
  }
}

If you look at the Swift generated interface for NSViewController, the method addChild is defined in an extension. So the extension in my subclass is overriding a method defined in an extension in its parent class.

Presumably this is allowed because NSViewController is an Objective-C class. Attempting to use the same pattern in a Swift-only class yields a compiler error.

But I've found very little documentation on this behaviour and was curious:

• What are the rules for overriding a method in an Objective-C class?
• viewDidLoad, viewWillAppear and several other NSViewController methods are not defined in an extension whereas most others are. Does it matter if my subclass overrides them in an extension or not?
• I assume the @objc decorator is playing an important role in this. Is there any documentation out there describing how @objc affects override such that it us allowed in extensions? Or is this behaviour supported because NSViewController is ultimately an NSObject?

macOS 10.15 Beta 5, Xcode 11 Beta 5