I haven’t measured the square footage of the planters I use but I’m sure it isn’t more than 50 square feet. Still, I get enough for myself and maybe a neighbor or two.
The books that I took those estimates from were a comprehensive and self sustaining program. The crops selected would provide all the nutrients that were needed including protein.
In addition, the intention was to be able to grow/save seeds and replenish the soil. Approximately 60% was high biomass such as grains to ensure suffiecient compost, and 10% was dedicated to crops for sale.
So less area would be needed for just growing some veggies to eat. The sq. foot gardening method, for example states that 1 16 sq. ft. bed of 4x4 is sufficient for one person to grow their veggies.