The concern boils down to whether the studies claiming there are no food deserts are classifying grocery stores correctly. The correspondent claims that the coding scheme lumps in convenience stores with grocery stores. Two commenters (1, 2) argue this objection is faulty: that the coding rules make sense.
Well…at least now we’re arguing about the right things! And using data as the basis for argument. (Though note a lot of the commenters at the post seem to be relying on personal anecdotes.)