Can't traceability be implemented with digital signatures? WhatsApp could sign my messages with my encryption key and always forward messages with their origina

Can't traceability be implemented with digital signatures? WhatsApp could sign my messages with my encryption key and always forward messages with their original signatures as metadata. Wouldn't that implement traceability? I guess that's the "fingerprint" this article alludes to. I don't see the technical limitation, just a policy one. WhatsApp wouldn't need a centralized store of all messages.