Fruits are actually not very necessary and only good in moderation. The only real benefit is the micronutrients which are prevalent in many vegetables, without the sugar. Eating too much fruit can result in overconsumption of sugar leading to insulin spikes, harmful if you are pre/diabetic.
I've been wondering how often there are examples of isst vs. ist being misunderstood in speech. I listened and thought "The doctor is no fruit" was weird, but maybe not too weird for Duolingo. Maybe some kind of "The doctor is a fruit" is some kind of insult. What do I know. The "correct" answer sure made more sense. Interestingly, Duolingo did not mark "is" wrong in this case.