Translation is based on meaning, not words.
In this case, the English "the fruit(s)" or "the pieces of fruit" often carry the same meaning. Most often we see this when asking for a fruit, when there are many. "May i have a piece of fruit?" Having the same meaning as "May I have a fruit?" And is the former is more common than the latter. Further, if people don't use the word "piece" then tgey still don't say that latter phrase often but rather ask in plural "May I have some fruit?" Even though they mean 1 fruit.
I don't get why DuoLingo translates "Koni gerpi syri issi." by "Those are good fruits" instead of "Those fruits are good". To me, "Thode are good fruits" would be the translation for "Koni syri gerpi issi". It's not the first time people are commeting on word order. Any thought on that?