I think the deal is you marry the strong crypto with a human mediated security process which provides high confidence the message sender maps to the human you think they are. And even if they are, they could be a narc. Nothing in strong crypto prevents narcs in whom ill-advised trust has been granted from copying messages they're getting over the encrypted channel and forwarding them to the man.

And even then, a trusted participant could not understand they're not supposed to give their private keys out or could be rubber-hosed into revealing their key pin. All sorts of ways to subvert "secure" messaging besides breaking the crypto.

I guess what I'm saying is "Strong cryptography is required, but not sufficient to ensure secure messaging."