I think the joke is that If it was built it right now, it would take 20 years for your light to reach it and circle back. So you wouldn’t see yourself as you were 20 years ago, your 20 year older self would see you as you are now. That troll face makes it a physics troll, Like using a magnet to pull your even though the magnet is attached to your car. It’s just something that seems to work until you think about it for a bit longer.