Gardening

As others have said, this article is not very accurate. Annual crops produce over a short window, so one would need to have successive crops lined up in order to keep the space productive. Growing something to get only one harvest is a very poor return on investment. If one wants to survive without depending on "the system" at all, then trying to do so outside of the equatorial zone is living life on hard mode.

Near the equator, one could survive on only bananas for a while, and that would take a small fraction of a hectare, probably about as much space as this article talks about, but realistically, eating only banana long-term is not feasible, and growing more variety requires more space. There is also the feast-or-famine issue if the gaps between harvests are too long. Preservation of the harvest is time-consuming and requires infrastructure that not everyone has (e.g. refrigeration). Living in a neighbourhood where everyone is growing food in order to survive would allow for trade, and so each individual/household would not need to diversify their food production as much, and someone's excess that they cannot preserve could fill someone else's harvest gap, reducing the total amount of land that each requires. Ideally, that's the way to do it, and some people are trying. Tree fruits make the most sense as staple foods, since they become self-maintaining after a few years (other than pruning to control size), and in a sufficiently diverse food forest ecosystem, the trees won't deplete the soil or invite plagues, so they don't require externally-produced fertilisers and -icides. With enough different species and a fairly non-seasonal climate, it's possible to grow enough fruit year-round, with some high-calorie staple(s) always in season.

But lettuce and lima beans? Good luck with that.

I think the math in this assumes everything is being sown directly into the ground, which for a lot of things is the least space-efficient option.

You can hang-grow tomato plants, vertical garden stuff that doesn’t need much soil, and build potato towers to reduce the footprint (25 sqft turns into 5 sqft pretty easily), and leaf veg/herbs grow very well in window planter boxes or pots (with or without supplemental light). You can also grow mushrooms for protein and those hardly take up any space, and can produce very prolifically (and are mostly grown inside anyway).

Lots of options to reduce the actual space needed to grow stuff for self-sustaining food.

Although I’m honestly an advocate for either growing in soilless (cococoir) or hydroponic, if you have the space to have those inside. A single tomato plant is enough for most people, and a couple bell pepper or other self-pollinating plants (easier than pollinating them yourself with a paint brush). Year round fresh foods are so worth it, and a lot easier than canning everything.

I'm sorry but this article is ridiculous.

-Proteins. If you’re going to survive on a garden, you won’t be eating meat.
...
Growing lima beans on poles will require about six square feet of garden per person.

Like for a meal or two. Not for a year. Not to survive on.

I've grown all kinds of things. Worked at community farms. Been hired to grow fruits and veg for other people at their homes as a side thing. Pruned and managed small fruit tree orchards.

A 2 foot by 3 foot patch of beans takes 60 to 90 days to grow, and that is not going to feed anyone more than once or twice. Whoever wrote this does not know what they are talking about. Like at all.