The docs you linked to are for class components (`this.setState`), not function components using hooks (`useState`).

From React's "Using the State Hook" docs:

> However, unlike this.setState in a class, updating a state variable always replaces it instead of merging it.

https://reactjs.org/docs/hooks-state.html#tip-using-multiple...

The `extendState` method gives you the same convenience of `this.setState` but with hooks in function components instead of class components.