The biggest problem I have is his overlooking what caused the cancer. Tell me is the flu a disease? What about a cold? Well both of these are caused from viruses just like the vast majority of cancers. Yet I would not call influenza or a cold a "defense mechanism", they are diseases caused from microbes just as most cancers.
Cancer does not serve any beneficial purpose to the body. It leads to tissue damage and death. How is this a "defense mechanism"?