"Fruit" is the singular. Well, sort of - it's a mass noun in English and so the singular actually refers to a general and unspecified quantity.
An unmarked Klingon word does not specify whether it is singular, plural, or a mass noun. Thus the word naH could legitimately be translated as "fruits" in some contexts. However, here it is just "fruit", so I'm not sure where you got the idea that it was plural in this case.
One of the English "best translations" is vegetables which could just as easily be a vegetable or the vegetable. naH vISop can be I eat vegetables, I eat a vegetable, I eat the vegetable, or the same options with fruit.
Klingons maybe think it's bizarre to classify foods that way. "So if it's a leaf, or back in primitive times when food was grown in dirt, it was the part of the plant that was under the dirt, it's a vegetable, but if it is a part of the plant that grows above ground, and has seeds right in the food, or it used to have seeds before they were genetically engineered out, it' s a fruit, except if it is one of this list of foods that fit that definition of fruit, but are traditionally called vegetables, plus there's this special exception the tomato that humans make jokes about because most of them know it's a fruit, but they still think of it as a vegetable?" Then you tell them fruit are the sweet ones, sweet not being a flavour Klingons seem to make a lot of fuss about, so then Klingons think the lemon must be a vegetable.