Which executor will run the completion handler of an `@objc` actor method?

Let’s say I have the following code

@MainActor
@objc
class MainActorClass: NSObject {
  @objc
  func myMethod() async throws -> Int {
    return 1
  }
}
@implementation MainActorClassOwner {
  MainActorClass *_mainActorObject;
}

- (instancetype)init {
  self = [super init];
  if (self) {
    _mainActorObject = [[MainActorClass alloc] init];
  }

  return self;
}

- (void)callMyMethod {
  [self->_mainActorObject myMethodWithCompletionHandler:^(NSInteger result, NSError *error) {
    NSLog(@"Result: %ld", result);
  }];
}

@end

Which executor will run the completion handler? When I tried it, the completion handler was run on the main thread. Is that guaranteed? If so, is it documented?

(CC @Douglas_Gregor who authored SE-0297)

Anyone? :smiling_face_with_tear: @Joe_Groff?