It’s a chain of trust, you have to trust the whole chain.

Including the entire other side of the conversation. E2EE in a group chat still exposes the group chat if one participant shares their own key (or the chats themselves) with something insecure. Obviously any participant can copy and paste things, archive/log/screenshot things. It can all be automated, too.

Take, for example, iMessage. We have pretty good confidence that Apple can’t read your chats when you have configured it correctly: E2EE, no iCloud archiving of the chats, no backups of the keys. But do you trust that the other side of the conversation has done the exact same thing correctly?

Or take for example the stupid case of senior American military officials accidentally adding a prominent journalist to their war plans signal chat. It’s not a technical failure of signal’s encryption, but a mistake by one of the participants inviting the wrong person, who then published the chat to the world.