But obviously not all apps can work without a runtime (server, edge functions, etc). So this must be meant for a niche type of app. What type of app is that? Who is this for?
You're totally right. It's not meant for all apps. Many apps cannot be implemented without logic in the backend.
I created a sample app to showcase a good example of an app that can enjoy this architecture:
https://github.com/jaydb-cloud/jaydb-kanban-demo