React Server-Side Rendering Memory Leak

The code below according to the link react says that:

Unfortunately, this can cause memory leaks for server rendering (where componentWillUnmount will never be called)

// Before
class ExampleComponent extends React.Component {
  componentWillMount() {
    this.setState({
      subscribedValue: this.props.dataSource.value,
    });

    // This is not safe; it can leak!
    this.props.dataSource.subscribe(
      this.handleSubscriptionChange
    );
  }

  componentWillUnmount() {
    this.props.dataSource.unsubscribe(
      this.handleSubscriptionChange
    );
  }

  handleSubscriptionChange = dataSource => {
    this.setState({
      subscribedValue: dataSource.value,
    });
  };
}

I cant understand how this can be a memory leak in server-side. For example, lets say we have this code which is rendered server-side and the ExampleComponent contains the memory leak.

import React from 'react';
import ReactDomServer from 'react-dom/server';
import App from './components/index'

const serverRender =  () =>{
    return ReactDomServer.renderToString(<ExampleComponent />);
};

export default serverRender;

When this returns to the client, the rendered components are not attached to anywhere and are ready to be GB collected. So why is there a memory leak?

this.props.dataSource is something external and can live longer than the component calling subscribe. handleSubscriptionChange will be referenced by this.props.dataSource. Also the component itself might be referenced by this inside handleSubscriptionChange. So GB won't clean up ExampleComponent at all.

Since componentWillMount is deprecated you probably should not worry about these details and just use componentDidMaount.