"Das sind doch keine normalen Äpfel!"
Who says 'Those are no normal apples' these days?? I said 'Those are not normal apples' and got it wrong.
In English, using "no" this way instead of "not" is not uncommon. It does add a certain emphasis.
"Those are not normal apples"--They're off the standard. I don't like their looks.
"Those are no normal apples" = "Those apples are weird!"
To me it just sounds old-fashioned, and I would never say it that way. Regardless, I have no problem with 'Those are no normal apples' being an accepted answer, as long as 'Those are not normal apples' is accepted as well.
I agree with Soglio, sounds fine.
That said, it seems like Duo erroneously wants to translation the word 'doch', which is why neither 'no normal' (the answer I put) nor 'are not normal' are accepted, but rather 'Those just (who knows where the just came from) aren't normal apples'.