I was responsible for maintaining a large backbone app over the course of 10 years. The main thing to know about Backbone is it’s not intended to be a fully fledged framework like React. It’s more like jQuery in the sense that it has utility, but in order to use it in a way that is easy to maintain over time, you need build your own layer of abstraction on top of Backbone to make things work the way you want. React h…
Neither is React. The built-in state management was meant for concerns relevant to only that component, not your entire app's state - people just realized it worked well enough.
Back when React was new, one of the ways to use it in more complex apps was with Backbone as the data store: https://www.npmjs.com/package/react.backbone
People weren't just using this as a migration path, before Rudux became the clear winner of that early React era people would reach for this combination (though possibly not with that specific library, I don't remember the details about using them together).