No one is saying don't look out! We're just discussing what to do when you've spotted someone, but they are about to go into your blind spot (because they will be behind you), and you're not sure that they have spotted you. And if they haven't spotted you, there maybe a risk of a collision.
No one is suggesting the radio is an alternative to a lookout. It's just that a lookout is not very effective for dealing with traffic catching you from behind.
dp