It would still need to talk to your email provider's AMP cache, presumably, though, unlike a normal email which you download once and then do not need to reach out to the server for again.
I don't think there's a need for an amp cache with amp4email. The privacy concerns that make an amp cache necessary don't apply in the same way, only the security concerns that require a safe html subset. So an imap client could cache the amp js locally, and render dynamic content by querying the sender directly.
I could be wrong though, but I believe that works.
I could be wrong though, but I believe that works.