It's not entirely removed from objective reality. Firstly, there is no such thing as objective reality for humans. Secondly, the principles that underpin this scenario are highly active in the world today. Things like specialization, efficiency, division of labor, etc.

Not too long ago my neighbor cleaned the carburetor on my lawn mower, and I fixed his router config in exchange. He would have had to spend tons of time figuring out how to do it himself, but it took me 5 minutes. I could have cleaned my own carburetor, but he already had the cleaner and had freshly done one, so it took him 5 minutes but would have taken me 30 minutes plus a trip to the store. The same principles from the example are at play here.

And whether Person B can get enough protein from coconuts is entirely irrelevant. All that matters in this scenario is that Person B was concerned about it and wanted to trade.