It doesn't, actually. It says most people who are in food deserts are in urban areas. The vast majority of food deserts, however, are absolutely not in urban areas.

Even if it were true, it still only affects 13 million people. There are 330 million in the US, so it's a non-issue with regard to our obesity problem.

OK, point taken re most people vs most food deserts

In any case, I am talking about the availability of items in response to the parent's obviously absurd implication "every food market I have been to sells X, therefore every food market sells X".

I am using food deserts as a counterexample since definitionally these are regions where certain items are hard to find. I know (hope?) that the person I am responding to likely doesn't believe every grocery store in the United States carries plain yogurt, but I also know that people here often forget that not every place enjoys the same level of choice that is enjoyed in places like the SF Bay Area

I truly don't understand why you are bringing up obesity, this feels very remote to what is being discussed.

I'm bringing up obesity because the overarching topic is UPFs, which are a primary cause of obesity, which is the main reason they are bad.