I think he was referring to a cryptographic signature, possibly using the "web of trust" to get the key. I'm not convinced we need central authority to solve this.

people at my org were gleeful when they learned they could hook LLMs into Slack. Even if we had some reliable, well-used signature system, I think people would just let AI use it to send emails on their behalf.

Same way security cameras prove that they are authentic camera recordings that have not been modified. If modified, the video will no longer match the signature that was generated with it.