I wouldn't call it "major" because (1) you refuse to assign a probability to an event I stated I thought was likely;

It puts an upper bound as I said. Plug the specific conditional I calculated into Bayes theorem and see what happens. Or look at a special case: suppose conditional on the diet not being harmful, Roberts had a 50% chance of dying before 80; now, what is the maximal amount in terms of odds or decibels or whatever that you could ever update your prior upon observing Roberts's death assuming the worsened diet risk is >50%? Is this a large effect size? Or small?

(Now take into account everything you know about correlations, selection effects, the plausibility of the underlying claims about diet, what is known about Roberts's health, how likely you are to hear about deaths of diet gurus, etc...)

(2) the main point of your calculation was pretty non-controversial and even without a calculation I doubt anyone would seriously dispute it.

One would think so.