They're different things. Semantic is the UI framework that you can use from React (in fact, there is a react-semantic-ui npm) or other.
React will deal with lifecycle of objects on the screen and state machines to mutate them, but the actual look & feel of the widgetry is defined by whatever ui framework you decide to use (bootstrap, semantic, blueprint, etc).
React will deal with lifecycle of objects on the screen and state machines to mutate them, but the actual look & feel of the widgetry is defined by whatever ui framework you decide to use (bootstrap, semantic, blueprint, etc).