You go read the article. The deciding factor is not whether the message has been delivered, but whether WhatsApp servers report to the device that the message has been delivered. There's nothing stopping them from claiming that no messages have been delivered and thus recovering all messages (as long as they had been preselected for false delivery reports) despite true delivery status.

It is in fact all messages. They can simply not deliver the first message and force a resend record that mesaage. Afterwards force again a resend with the old encryption key and deliver that mesaage. No one would get a notification.

Can't they (WhatsApp) simulate a user being offline?