under the impression that eating on less than $3/day was not particularly typical beyond poverty

You're right that no one else I surveyed was spending that little, so it is unusual. I was just surprised to see expected savings larger than what I'm used to spending.

Your link says that "typical family of four in the United States making the median household income would have to double its food expenditures in order to eat what USDA nutritionists consider a healthy diet" but bases that claim on the cost estimates from the Thrifty Food plan. The problem is that these estimates come from a ridiculously limited optimization process where they group food into about 60 categories ("whole fruits", "orange vegetables", "whole grain cereals"), then assign costs and nutrition information assuming that people are eating from the category in the proportion people do on average. So if the "whole fruits" people tend to eat are 40% apples, 35% oranges, and 25% bananas, then the cost will be a weighted average of those three. Once they've assigned costs and nutrition, they run some optimization to figure out how cheaply one can get the needed nutrients. The problem with this is that they can't say "eat bananas to get more potassium" because the only knob their optimization thing can tweak is the "whole fruits" one. This means that if a category is not homogeneous in terms of either nutrition or cost, they'll not be able to optimize well.