The app makes sure only the people it thinks are in the channel get the keys. If you turn on the security notifications you can see how it works, you get a message when someone gets a new phone, that indicates they got a new key.

You can even check if it matches up if you meet in person or if you communicated using another medium.

Right, if the invite link is publicly available anyone can join.

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.

These new people cannot access older messages. e2ee

There's also the fact that any of those messages can be reported for doxxing or screenshotted to show proof that they're doxxing. Also, Telegram group chats are encrypted, just not E2E, so they've also likely been reported by people who visited them. They are public, after all. (Which seems like a puzzling approach when you're doing something illicit but that's a whole other matter.)