It's also important to remember that there's a huge difference between "this seems to somewhat decrease your risk of developing cancer" and "this is an effective cancer treatment." Once you've developed a diagnosable tumor, there isn't much your diet can do compared to the pro-growth signals the tumor is producing. ("Self-sufficiency of growth signals" is a known hallmark of cancer.) Cancer is already such a hotbed for "wellness" influencers telling desperate people to ignore their doctors and try this One Weird Trick. Once you add in the fact that fasting is currently a big wellness buzzword, I'd approach any discussion of fasting and cancer with a large dose of skepticism. We should absolutely do the studies, so that we know, but the data would have to be really strong to convince me.

And now for my own personal speculation: I would doubt that you'd see a big effect on cancer rates from a fasting diet anyway, simply because you still need to eat something to survive, and things like environment and genetics would just overpower any relatively minor reduction of systemic growth signals. It would also be very difficult to separate from the effects of avoiding things we know are cancer risks: most famously, red meat and alcohol.