I think this used to be true. Now one problem is that a Signal message goes through this whole forward secrecy protocol, but the receiving device has some probability of uploading it to the cloud with a static key that never changes.
You don't have to enable the Signal backups feature, but you have no way of knowing whether the recipient of your messages has. One person in a group chat with that enabled will undo all of the forward secrecy you're describing.
You don't have to enable the Signal backups feature, but you have no way of knowing whether the recipient of your messages has. One person in a group chat with that enabled will undo all of the forward secrecy you're describing.