Here is the actual paper: https://www.jci.org/articles/view/169671
What happens is that the antioxidants, including vitamin C, stimulate the tumors to grow blood vessels.
The tumors with better blood vessels grow faster.
So the antioxidants that are good for you are also good for tumors. Therefore they must be avoided whenever cancer is identified and you have to take medication that is bad for you, but hopefully even worse for tumors.
Some anti-tumor medication has precisely the purpose to prevent the growth of blood vessels. This may stop the growth of the tumor, but it is also obviously bad for your other blood vessels, so it can be used only to gain time until some other form of treatment becomes feasible.
Is that basically what chemotherapy is?
I think the consensus is that supplements aren't useful as long as you don't have a deficiency (which is pretty uncommon in the west if you aren't eating particularly bad).
Consuming too much of anything that is useful in small quantities, is either useless or harmful.
When restricted to the same small quantities that are normally available from natural sources, there are no significant differences between vitamins from supplements and from their natural sources.
There may be good reasons to use supplements, as long as their dosage is right. For instance, eating one raw red bell pepper per day will provide enough vitamin C for a human. However, the same amount of ascorbic acid powder costs ten times less where I live, in Europe. So either taste or budget may make the supplement preferable, which is fine, unless a much greater daily intake is used than it would have been provided by the bell pepper.
I was also just making this point of clarification for other people, and not implying that a reasonable reading of your post would determine you thought otherwise.
What an odd nitpick, especially as it's incorrect.
> fuel, verb, 2: support, stimulate [1]
By improving the blood flow, it supports, stimulates, or fuels the growth of tumors.
As a nutrient/oxygen delivery network, in an adult human it should not be a problem unless one's injured I imagine? That's because the vessel network is fully developed, but I don't know if my intuition is correct on this one.