I'm not aware of a framework that has first-class support for animating between application states, and I've used all the most prominent ones. But regardless, the fact that it's leveraging a native API is significant; it means (a) the browser engine can optimize for it, (b) less code needs to ship to the browser to use it, and (c) native behavior means a standardized API for a complex task.

Do you have an example? My understanding is that without browsers doing the tweening between visible states, it would require complex DOM manipulation and animation to make it look good. The native API is an essential part of making this happen for the web.

I think you need to read the API docs again. View transitions as a concept are not new but the View Transitions API offers functionality not previously available in browsers.