No, that's not about sugar. The body needs glucose to function. It is produced from carbs (and other sources) in your food.

Yes. That's why keto is used by some cancer patients.

Not all cancers are glucose-sensitive though.

See the Warburg effect - https://en.wikipedia.org/wiki/Warburg_effect_(oncology)

Also, water-only fasting.

Keto is a successful form of cancer treatment mostly for brain cancers. Low carb/keto haven't shown dramatic benefits for cancer treatment (like most treatments, it helps some people) or for cancer prevention.

Exercise seems to fix cancer at the source as indicated by studies showing it dramatically reduces cancer risks. So IMHO restricting carbohydrate is trying to solve the consequence of excess energy through diet whereas exercise solves it at the source.

Exercise causes a variety of changes in the body beyond just altering blood sugar levels. Not all cancers are glucose driven. There is likely a larger grouping of cancers that are facilitated by excess energy in the body (glucose being one form of that), all of which might be helped by exercise. But exercise has many other downstream effects.

Rather than restricting carbs, an approach to requiring a bout of exercise before eating would be much more in line with evidence.