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.

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.

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.

Those are not some private groups of 5 guys. Those group are available to millions of users.

You can still tell but of course there shouldn’t be an expectation of privacy with that many users.

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.)

They can see the title and possibly the picture, though. (People who know this use inconspicuous names and pictures)