You have to distinguish between these two statements:
a) There is research that indicates that there is a benefit to fasting
b) The research indicates that there is a benefit to fasting
Statement a) is true; but b) is what you said.
Going from a) to b) is the hard part. It's not enough to know what some research says, you need to place it within the broader context of the field of knowledge before you can make declarative statements of benefit. And cancer is an incredibly complex and fast-moving field of knowledge.
The oncologists I spoke with were aware of research into voluntary fasting, but on balance recommended against it. One reason is that unlike with a healthy person, ending the fast may not be under the patient's control, depending on how they tolerate their chemotherapy. Chemotherapy ends up looking like intermittent fasting for a lot of patients anyway.
Chemotherapy is also cumulative, which means that decisions that appear beneficial in early rounds may end up limiting the total number of sequential rounds that a patient can tolerate.
If someone is not habitually fasting, adding it on top of chemotherapy adds a significant confounding factor to a process that is already hard to predict.
And aside from voluntary fasting, there is a lot of evidence that patients who can eat well and keep their weight up have better outcomes from chemotherapy than those who can't/won't eat and lose weight.
This is what I was told by people who spend their lives studying the treatment of cancer.