Her signature can be tied to her (that's the point of the article after all), and Bob can supply the evidence for that by decrypting it. If there are no other

Her signature can be tied to her (that's the point of the article after all), and Bob can supply the evidence for that by decrypting it. If there are no other ways for Bob to have received that signature, then that's sufficient to establish communication. If Bob could have gotten it from Charlie because Alice did the same dance with him, then indeed Bob can't prove anything. But then Alice can't be sure Charlie isn't forging messages to Bob, and Bob can't trust Alice sent them in the first place. But as I said I agree which message can't be reliably determined based on your scheme.