I'm also skeptical that adding foods/vitamins/supplements will prevent cancer.
However, there is one area that adding things seems to be very beneficial, and that is adding things that promote regular bowel movements.
There have been studies of heavy coffee drinkers and their reduced risks of colon cancer. Everyone is looking for some mysterious phenol or antioxidant from the coffee as a magic elixir, but I think it should be obvious that coffee as a laxative is the agent here.
There's also the age-old rule of thumb: an apple a day keeps the doctor away. It's not the apple, it's the fiber.
Regular bowel movements are their own reward!
But diet is a complex system. So while there might not be any magical "direct" benefit from eating a certain fruit or vegetable, they could still produce a significant positive effect.
Perhaps the biggest contribution to preventing cancer that fruits and veggies make is to curb negative behavior. They might replace sugary foods or regulate cravings or satiate (all interrelated). And then, this could feed back into promoting positive behavior: more energy which encourages more rigorous exercise. Which boosts the subject's mood and increases their sense of well-being. Which feeds back into their dietary choices. Etc.
Obviously there's benefits, but I maintain that adding fruits and veggies to an otherwise awful lifestyle is akin to putting lipstick on a pig. Eat all the fruit you want -- if you smoke, you're not fooling your body.
My conclusion was that cancer is best prevented by removing negatives from life, not necessarily adding positives. ie avoid anything from stress to pollution to sunburn to obviously smoking. Interestingly, organic food doesn't seem to matter in the cancer equation, given the data we have.
Yet there's one thing you can ADD to reduce cancer: exercise. Preferably moderately hard to strenuous.
[1] https://blog.priceplow.com/vegetables