It's not really as bad as you suspect. A lot of homesteaders target food self-sufficiency on a quarter acre; while there's some debate [1] over whether that's possible, pessimistic estimates for the amount of space needed to feed a skilled organic gardener's family of 4-6 top out at about an acre. A 100x100m plot is about 2.5 acres.

The bigger problems are a.) skills and b.) labor. Homesteading methods basically require full-time gardening by people who have put significant efforts into maximizing yields, intercropping, planting multiple harvests, growing vertically, etc. The advantage of our mechanized farming system is that we can feed the population with 1.4% of the population.

[1] https://www.theseasonalhomestead.com/the-truth-about-self-su...

I was hoping your figure would be wrong and it is, but the situation is actually worse than than. 1.4 billion hectares of arable land divided by 7.9 billion people yields 0.18 hectares per individual, which is actually a 18×100m plot per person, 5 times less than your figure. And that's before conservation areas, wasting arable land to build suburbs, etc. Looking into national figures is interesting, showing that China has less than half than amount, but also shows that something is off with the figures themselves. Extremely arid and sandy Niger has more than 5 times the global average, which doesn't pass the smell test. I was personally shocked by the sheer unsuitability of the land for growing crops in the most fertile regions of that country, much unlike the lush southern areas of Mali for instance. Forested areas don't count as is clearly seen from low-ranking Brunei, which is 98% primary forest with extremely luxurious landscapes. Papua New Guinea is mostly self-sufficient and yet also extremely low, clearly demonstrating that agroforestry systems aren't accounted for at all. I'm sure other absurdities are to be found. Lies, damned lies, and statistics…

[1] https://www.nationmaster.com/country-info/stats/Agriculture/...

That is a super fun fact. Not that it changes the calculus, but I would be curious what percentage of human calories come from water sources (fishing, seaweed, lobster, etc).

Luckily ruminants do not need crop-suitable land to thrive.