That’s the whole point. Her actions were driven by irrational fear due to the cancer her mother suffered from; although Marcheline Bertrand did have breast cancer, she died from the ovarian cancer.
Her fear was not irrational. Her situation is also very different than the vast majority of women as she tested positive for a BRCA genetic defect.
Her chances of breast cancer were well over 50%. That is more than twice the female population as a whole. Her risk of ovarian cancer is also very high.
BRCA defects are found in only a small percentage of the poplulation.
She carries a BRCA genetic mutation, which can lead to both breast and ovarian cancers. I tested BRCA positive, too, but I didn't find out until after I'd been diagnosed with cancer.
For the record, the article and its headline are not clear enough. At the end of the article, though, there's this line: ‘Double mastectomies are not routinely offered to women with breast cancer in the UK unless they have a family history of the disease or a high risk of recurrence, as outside of these groups there is no evidence to suggest this would be of benefit.' IOW, within those particular groups, there is a benefit.
To be honest, it's been known for years that, statistically, there's no difference in survival rates for lumpectomy with radiation vs. mastectomy... unless the woman isn't eligible for lumpectomy because the cancer is multifocal, for example. For years now, surgeons have been routinely advising patients to have lumpectomies, except when the patient is BRCA-positive (like Jolie) or the cancer is multifocal, etc.