The picture in the article is Berlin. Thought you should know.
Science
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CTRL+F "plant based"
Ah, they didn't get into the broader topic of food security.
The optimal crops for urban agriculture were peas (normal climate), and sugar beet/spinach (nuclear winter); while those optimal for industrial near-urban production were potatoes (normal climate), and wheat/carrots (nuclear winter). Urban agriculture could feed a fifth (20%) of the population. At least 1140 hectares of near-urban cultivation could make up the shortfall. Another 110 hectares of biofuel feedstock like canola (rapeseed) could provide biodiesel to run agricultural machinery without fuel trade. Significantly more cultivated area is needed in nuclear winter scenarios due to reduced yields.
Plant-based is the default.
The "medium sized city" in question has a population of less than 100,000 people. I think most of us would call that a small city at most. The city is also has a temperate climate meaning year round growing seasons are possible. Yet still the city would need a lot of farm land around.
In short this doesn't apply to you. If you are a "survivalists" you best preparation is to the ability to get to a rural place where you will be welcome. They will need extra labor and they have the crops already, but if they don't know you they may refuse to let you in (or only allow you in as a slave?) If you can't get out (which you may not be able to!) knowing your neighbors is a good second best, if neighbors looks out for each other they can find a lot of ability to survive and there is some hope that you can grow food in your city.
No qualms about medium sized, but I think you missed the mark on your temperate climate description. Temperate climates are characterized by four distinct seasons. Not much you can harvest in winter.
Tropical humid climates are stable year round. As long as it can produce there, it can produce all year. For instance, grapes normally yield once per year on temperate climates, but twice on tropical ones.
Four seasons yes, but not cold enough for snow - coldest normal temperture would be -3C. Different plants grow than summer but you can get something if you choose your winter plants. Or maybe you only lose out a month of growing season.
Not to "uhm, acktshually", but technically New York is temperate (according to Koppen, temperate subtropical, which leads me to question his wisdom), and it snows quite a bit there.
But I see your point. There are plenty of people living in less favorable climates agriculture-wise.
The city in question is in new zealand. While new york may be temperate (though it doesn't show up in the maps I see - might be looking at maps on my phone) that doesn't mean it is really comparible.