From a calorie perspective I don't think that's true. If you grow all potatoes which are one of the more efficient calorie/acre crop you have approximately: 90 days for 1 crop at ~15e6 kcal/acre. Nimitz class aircraft carrier has a surface area of ~1 acre and a crew of about 6000 people requiring ~10.8e6 kcal per day. So after 90 days of growing potatoes you would have about one days worth of food, or enough to feed ~66 people continuously. Of course you could always fish...