No comment about how secure the below is, but it seems trivial to not need data on the message itself.

The only data that needs to be stored by WhatsApp is `Sent` and not the message itself, which can be stored locally by the Messenger.

[Messenger] <--> [Sent Succesfully?] <--> Recipient

When the Recipient receives a message - they alert WhatsApp that the message has been received which can then alert the Messenger that the message has been sent. If it comes back with a failure, the Messenger will attempt to send the message again in `${Time}` until it sends. The message itself never has to hit WhatsApp's servers.