I think I understand what you're suggesting, but keep in mind that in the Wikipedia example the database is largely static (the server decides when to update), while a messaging app needs to support users updating the DB. There are a lot of leakage scenarios that need to be taken into account (like no push notifications).

Spiral has a video[1] where they dive into some of the details.

[1] https://youtu.be/T7RDEEJ5vQg