Yep I actually thought exactly the same, but only a month or two ago in a discussion on another Duolingo course several other native English speakers from different countries convinced me that they actually do say "three fruits" where I would only ever say "three pieces of fruit".
American here. I would absolutely say "three fruits" to refer to either 3 pieces of fruit or 3 types of fruit, although it may be ambiguous which one I mean in context.
In the lesson I wrote "mangoes are fruit" which to me looks as grammatically correct as "mangoes are fruits", although both were rejected.
What worries me is not fruit but "The" in translation. In Hindi sentence nowhere does it imply that we are talking about any specific mango. The lesson before I was doing a similar sentence with an apple? and the correct translation was An apple is a fruit. Here there was no option to choose "An" because there was none. Someone who makes does sentences is not consistent... As for me "The mango is a fruit" is not correct grammatically as we talk in general about mangoes , not any specific one, and yet transation "Mango is a fruit" was not accepted.
In English "A mango is a fruit" is an unnatural/weird expression. We would be far more likely to say "Mangoes are fruits". It's not a word-for-word literal translation, but it's always good for the course to translations that are things British/Aussie/Kiwi/American/Canadian etc, English speakers might actually say.
So I've reported that "Mangoes are fruits" should be an accepted translation.