Smoking is a symptom of being addicted to tobacco.

Obesity is a symptom of food addiction, which can tie into trauma, mental illness, depression etc. There are of course physical conditions like lymphedema that can affect it as well but usually it's tied together with food addiction.

Saying "people die of cancer not smoking" is like saying "people die of heart disease not obesity" ignoring how obesity and smoking are major risk factors for heart disease and cancer.

It's a statement that doesn't really mean anything.

That said I don't agree that body positivity is killing people. I think shaming the obese does nothing to help them and in fact makes things worse and harder for them. It's easier to make healthy choices when you have good self esteem.