I don't think this is nearly as crazy as you may think at first glance

Imagine if it was just a hidden (special) folder in an Exchange mailbox.

Voila, you already have a well-known and widely implemented and tested message syncing solution both for content and status (read/unread)

I assume Windows Phone worked the same way with its text message backup. When you'd set up a new phone it would take a while for your Microsoft account to finish syncing during which new messages would trickle into the Messaging app in real time. In fact if your old phone was still on WiFi new messages would show up on both. Still more advanced 15(?!) years ago than my Android today