That's just a theater. If anybody can join a group, there must be a way to distribute the keys to those who joined. Anybody can get a key then, including Whatsapp/Facebook.
But a Facebook employee would have to join the group and they wouldn't have access to older messages. It's not like with Telegram, where all the content, including old messages, is stored in plain text on the server.
You can even check if it matches up if you meet in person or if you communicated using another medium.