To be a pedantic shit, I wouldn't say React is a _UI_ library. React's deal at the time of its release was 3 things:

- You can mix js and 'html' in one component

- The unidirectional 'flow' of state

- The internal model / diffing so only components that changed would update the dom

It's a mix of a library (the 'html' elements) and an architecture pattern (unidirectional flow) imo.

I'd expect a UI library to be things like buttons, windows, UI components, etc.