Why 'JRootPane' is-a 'JComponent' in javax.swing?

As per below code, I see that, setLayout() is being called on a JRootPane object instead of JFrame top level container in javx.swing world unlike java.awt, because it is told that swing's JFrame is light-weight container compared to AWT's Frame.

public class TestGetContentPane extends JFrame {
   public TestGetContentPane() {
      Container cp = this.getContentPane(); // cp points to 'JRootPane' object
      cp.setLayout(new FlowLayout());
      cp.add(new JLabel("Hello, world!"));
      cp.add(new JButton("Button"));
      ......
   }
   .......
}

Any JComponent will be part of the layout set in Content-pane JRootPane which in-turn is a member JFrame or JDialog top-level containers. So, JRootPane looks like a container but not component.

/**
 * @see #getRootPane
 * @see #setRootPane
 */
protected JRootPane rootPane; //member of JFrame or JDialog

Interestingly, below code says that, JRootPane is-a JComponent.

public class JRootPane extends JComponent implements Accessible {}

My question is,

1) What exactly do they mean, when they say JFrame is light-weight container unlike AWT Frame?

2) Why JRootPane is-a JComponent? I feel, JRootPane should be along the class hierarchy where JFrame & JDialog sit under Container