I think that this advice stems from research that shows some lifestyle choices increase the chances of getting cancer. Personally, I think some of that research is a bit wobbly, but that's the current stance. Stories that came out last week suggest it just bad luck.
I agree that when you already have the disease and have tried to look after yourself, suggestions that looking after yourself will reduce your chances of recurrence are not helpful. Trying to boil it down to something you may or may not have done in the past will do your head in.