Are you sure about this? There is data to back it up in The China Study https://en.wikipedia.org/wiki/The_China_Study and mentioned in the Forks over Knives documentary. That's more about prevention, but it has a measurable impact on cancer rates, I don't see how that could be classified as no impact.

The book has been debated.

https://www.redpenreviews.org/reviews/the-china-study-the-mo...

If diet stopped cancer, then the patients who eat less would see noticeable effect. This has never happened, except for maybe Otto Warburg, but even that is disputable[1].

[1] https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP278810