I don't think nobody ever use "butir" to count oranges. I think "sebuah, dua buah jeruk" would make more sense.
What is the "butir" for?
It's a counting word, like lima orang guru (five teachers), lima ekor kucing (five cats), lima lembar kertas (five pieces of paper).
Why butir and not buah?
Dont get it at all.
So until this lesson came along "lima jeruk" would have been accepted, right? But would the omission of butir in a non-Duo real-life context be "wrong?" Or is it declared wrong here because I should have guessed the lesson maker's intent?
Saya punya lima jeruk. is CORRECT and SHOULD BE ACCEPTED