Email falls at the very first hurdle presented in the article (“No privacy whatsoever” applies just as well to email as it does to SMS).
I would even bet that any email leaks that most users would be afraid of are going to be caused by, in this order:
A) The intended email recipient leaking it
B) The email recipient/sender storing a copy of the email at rest (likely unencrypted, even if it was sent encrypted in transit).
C) An official email archiving policy from either the recipient or the sender's organization
And only at at the very bottom of the list "one of the intermediate mail relays being compromised and leaking the email in transit".
As for envelopes and wax seals--it's an interesting question. It requires a lot less technical knowledge for someone to open that envelope than to spy on messaging traffic. On the other hand, a lot more people have access to the messaging traffic and can bring scripts to bear on it at scale.
Actually, I disagree here.