This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The HttpSessionListener and HttpSessionAttributeListener are configured in the deployment descriptor. Therefore, even if a session attribute implements these interfaces, the sessionDestroyed() and attributeRemoved() methods will not be called on that attribute.
I don't know what the context that paragraph comes from, but I think it means it's not enough that the class implements the interfaces, it also needs to be registered in the web.xml in order for the listener to work.