Well, occasionally it's true. Cheerios are pretty good for you, as cereals go (for example). I imagine you're talking more about "fat free" on a package of Twizzlers. ;) (Of COURSE it's fat free; there's nothing in it but sugar!)
Perhaps the person who uses this phrase is using Michael Pollan's definition of "food":
"...You’re much better off eating whole fresh foods than processed food products. That’s what I mean by the recommendation to eat “food.�? Once, food was all you could eat, but today there are lots of other edible foodlike substances in the supermarket. These novel products of food science often come in packages festooned with health claims, which brings me to a related rule of thumb: if you’re concerned about your health, you should probably avoid food products that make health claims. Why? Because a health claim on a food product is a good indication that it’s not really food, and food is what you want to eat."
Using this definition of "food", the phrase "food advocate" makes sense (it has roughly the meaning that chained_bear gave, below). The problems start, though, when you assume that your listeners are using the same definition of "food" that you are.
Like the Lorax? Ha ha ha!! "I am the Lorax. I speak for the food!"
No, it's supposed to mean a person who supports/agitates for organic, small-farm-grown vs. "industrial" agribusiness-created (etc.) foods. I'm not against the cause (though I have my own views I'll refrain from stating here), but this phrase needs serious help.