I also received a postcard confirming that Informed Delivery had been activated for my postal address.
Maybe it would be good to also add in a postcard check where they send you a code to verify you have access to mail for the address anyway. Sure, a thief could too, but in that case there isn't much of an extra risk to informed delivery since you'd be compromised already.
I guess asking about credit report info is inadequate if people are managing to abuse this. One problem with that kind of thing is it's essentially security by obscurity, it's not deliberately created secret knowledge that can be trusted, it's stuff which happens to not be public and which only you should know, but there's no guarantee and you can't easily change it, right?
I definitely received the postcard though.
So anyone would've had a 1/16 chance of just guessing it right.