The carbon in food is not captured or emitted in any coherent sense here. The crops are grown (capturing the carbon in the first place) for the purpose of feeding people -- in the same way that modern American forestry for paper is functionally carbon neutral (ignoring transport and processing) because the trees are in equilibrium. The counterfactual of not eating the food results in fewer crops and basically the same atmospheric carbon dioxide.
Edit: if you only mean food transportation carbon, it seems impossible bananas are literally optimal per calorie.
From the end notes, I think the author's response would be that the carbon equivalent emissions come from the fossil fuel used in growing, fertilizing, harvesting, transporting, refrigerating, packaging, and so-on.
The larger point of the book is that specific accounting is messy, but if we proceed anyway we can get to the rough orders of magnitude that are more useful.
Edit: if you only mean food transportation carbon, it seems impossible bananas are literally optimal per calorie.