ReferenceError When Using dependOn in Webpack

Fairly new to webpack (sorry if this is kind of basic). From some searching, my understanding is that import declarations are unnecessary if you have the required packages in your dependOn in your webpack configuration. However, I get a ReferenceError: ReactDOM is not defined with the following config.

What am I doing wrong?

webpack.config.js

module.exports = {
    entry: {
        reactVendors: ['react', 'react-dom'],
        home: { import: path.join(__dirname, 'src', 'index.js'), dependOn: 'reactVendors' }
    }
}

In my index.js

// removed import React from 'react' and import ReactDOM from 'react-dom' up here;

ReactDOM.render(<p>Hello</p>, document.getElementById('demo'));