[upofadown]

>Systems that lack forward secrecy are by design incapable of preventing archives of eventually-plaintext messages.

That is not what is being claimed here. Unless you add extra security in the form of something like a strong unique passphrase for the archived messages then an attack that gets the private key also gets the archived messages. In general, if you have a more secure method for protecting the archived messages you could of used it to protect the private key. It is effectively the same problem.