[Cryptography] encrypted list mail, was IAB Statement on Internet Confidentiality

[John Levine]

>- Sender chooses a random message key; encrypts his message with a symmetric algorithm; signs it with his private key; appends the message key,
>encrypted with the moderator's public key.
>- Moderator checks the signature; decrypts the message key; adds a second level of signature to the (signed by the sender, encrypted) message; and,
>for each list member, forward the doubly signed, encrypted message, with the message key encrypted with that member's private key, to the list
>member.
>- List member checks both levels of signature, decrypts the message key, decrypts the message.

This sounds exactly like what Sympa does with S/MIME.  What am I missing here?

R's,
John