From a question on another lesson: 'hotha' is used when saying it as matter-of-fact, and is not necessary unless you intended to say it that way
This should accept both "The mango is fruit" and "The mango is a fruit".
For many native English speakers, "fruit" is an uncountable noun and not normally used with "a" or with numbers unless expressed as "pieces of fruit".
"Fruit" as an uncountable noun refers to a physical mass of fruit. Fruits, or a fruit, refer to types of fruit. Hence phrases such as "fruits of the forest".
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".
I'm Australian and I can't remember if the people saying they use "fruits" were American or British. I have a hunch they were American but I could be wrong. I also can't recall which course it was as I'm doing quite a few of them.