Agree that instanceof is wrong and discredits the article.

I'm not sure if this is what you meant, but here is how polymorphism along with the double dispatch pattern would work: NaiveActor#handleLifecycleMessage will need to delegate to the event subclass, which will in turn select the appropriate method on NaiveActor.

Example of subclass of LifecycleMessage:

        class ExitMessage implements LifecycleMessage {
            @Override
            void handle(BasicActor actor) {
               actor.handleExitMessage(this);  
            }
        }

In NaiveActor:

        @Override
        protected void handleLifecycleMessage(LifecycleMessage m) {
            m.handle(this);
        }

        @Override
        protected void handleExitMessage(ExitMessage m) {
            if (Objects.equals(m.getActor(), myBadActor) {
                System.out.println("My bad actor has just died of '" + m.getCause() + "'. Restarting.");
                spawnBadActor();
            }
            return super.handleExitMessage(m);
        }

It's been my experience that many developers resort to instanceof or enum type flags because they don't believe polymorphism actually works in real world situations such as this.