The issue with vegetarianism is that humans are not meant to eat just vegetables. Therefore, great care must be taken to get certain nutrients which are not found often in vegetables. Protein is the big one, but there are others.
However, with a properly crafted diet (an issue that most? vegetarians run into) you should be able to get everything. (possibly with the aid of supplements).
Cancer treatments typically deplete the body of protein and lean tissue, and most doctors agree that eating meat is just the fastest/easiest way to recover.
Its possible/probable a macrobiotic diet could contain enough protein and still be 100% vegan, but if there's another "imbalance" (say GMO Soy or Legumes which contain Estrogen) that could create issues he's had to deal with during his recovery.
Also, statistical outlook data for cancers, surgeries, etc, are based on statistically average populations. Vegan people deviate that from that population, and that could mean their outlook/treatment be different.
Jobs' whipple surgery drastically changed his digestive system. And because it's at least sort of rareish, I am not sure any good data exists for proper procedure on vegans who have undergone a whipple surgery.
In the fourth paragraph: "Jobs is a pescetarian (not a vegetarian or vegan as is often claimed) — although he does not eat mammalian meat, he reportedly eats fish from time to time."
My father in law has a variety of this. He lost something like 70-80 pounds before they got it under control, but has been doing well for years now.