This brings up an interesting “risk control” that one of my tech investors personally implemented with his family, in case a audio/video version of him ever asks to do anything crazy: secret passwords, agreed upon in person.
I notice every time somebody gets a new phone because it says "Your safety number with x has changed" but whenever I've spoken about it with friends, they have no idea what it means. An additional sentence could help here, such as "You might want to double check that you're really talking to x" or the classic "It's possible someone is doing something nasty". Although I understand that this would definitely scare a lot of people, maybe even push them into thinking Signal is insecure.
The UI really could stand to be more assertive about what they mean though.