The way the 18% figure is used in THIS article is extremely misleading/unhelpful. Thank you for clarifying exactly what they mean by that (the 5-6% thing puts it into perspective).
See also here: http://www.anh-usa.org/you-are-what-your-food-ate/
Vegetables and leafy greens are not the holy grail, because for this goes almost the same as meat. They can be full with pesticides and be nutrition deficient because of industrial farming.
Whatever you eat, if you want to avoid or lower your risk of getting cancer, eat only organic, non-processed food, ban sugar, and drastically limit the intake of fructose-rich fruits.
And for those who don't care, eat whatever you want, nobody stops you from doing so :)