You’re right, the value is derived from the fact that it is food, however the price value is able applied to the goods because of the standard unit (1 = 1). 

I believe the point has been blurred because of the examples being used to explain it. The initial comment and focus of the discussion was about the value of one asset comparative to another (finance tickers, XMR/BTC), in which case they cannot have value without being denominated in a standardized UoA like fiat. 

In terms of purchasing goods, I fully agree with you that it is subjective and more people should use Bitcoin as their preferred currency in circular economies. We are just not there yet.