Don't be mistaken, these mono-fruit islands are only temporary. The two did them from only 10 days. Mono-fruit islands are like Water Fasts that are only meant to be done for minimal amount of days needed. Dr. Graham creator of 80-10-10 does use both mono-fruit islands and Water Fasts for his patients, but usually from what I've seen recommends mono-fruit islands first to see if that resolves their issues before recommending a water fast. The reason is to avoid calorie restriction as much as possible.
After these temporary protocols for certain health conditions, eating a ratio style of 80-10-10 is what's recommended for staple diet.
All fruits and veggies have amino acids and fatty acids in them. From what I've seen from people who crave protein, fats, and salt on that style of diet, it's almost always because they weren't eating enough and getting enough calories and sometimes not eating diverse enough fruits of veggies, not because the diet is inherently deficient in protein or fats. Adding minimal amounts of animal products would guarantee you would never go under, as long as you're getting enough calories.