It’s important to read that paragraph very carefully. She is talking about preventative vaccine possibilities in the future. For preventative vaccines, you’d need to know the cause in most cases, and even if not, you’d need to know the antigens to target.
For example, Gardasil targets antigens associated with HPV, which, is the main cause of cervical cancer.
It will essentially prevent nearly all cervical cancer.
Not all cancers have a cause, in which case you’d still need to know the critical tumor antigen targets and escape antigen targets before any cancer formed.