Sometimes, Duo seems to be more literal than others. I think "His work did not bear fruit" and "His work was fruitless" is pretty interchangeable, even if it was not for "Arbeidet hans var frukteløs".
They are definitely interchangeable in a semantic sense. But "he had zero fruit" and "he was fruitless" are also the same in English semantically, but also worded different in English, so it makes sense that they're different in norsk. Just report that your answer should have been accepted and it will give them a chance to agree with you and add it. Remember that they can add correct responses. Can't think of everything the first time around!
In spanish we say something similar "su trabajo no dio frutos" meaning that his work did not give him anything rewarding :D
In Polish we often say 'Jego praca nie przynosi owoców' what is almost literaly translation of the norsk sentence.
BTW ,nie przynosi owoców' means 'doesn't bring any fruits'