If your only method of communication is a blockchain, a public key is who owns that public key. "Hello, I am q8oYflHjXyXj7Pgu /0R4fkOjFG83GTI8 2bmTfBkzRcLJXNiN

If your only method of communication is a blockchain, a public key is who owns that public key. "Hello, I am q8oYflHjXyXj7Pgu /0R4fkOjFG83GTI8 2bmTfBkzRcLJXNiN 4FuHE7Me71aWyTbk, let's agree on a symmetric key." is incompatible with the would-be impersonator supplying a different public key. (Assuming you're paying attention, I guess, but the original claim was that it was impossible, not that it was awkward and easy to screw up, since the latter is more or less true of practially all cryptography.)