There is a difference between your chat partner leaking it on purpose or just due to bad key management.

On #nostr your private DMs are public but encrypted. If one of both keys is leaked, everybody can read it.

Contrary Signal has a way more sophisticated encryption and the user does not need to bother with key management. Furthermore the chats are not public, so even if you could decrypt it, you first need access to the messages.

It’s a huge difference regarding privacy and security!