tl;dr hit the report button so the course creators can see what they missed. So as far as i can tell the app doesn't use a fancy algorithm or anything like that to determine whether a sentence is correct or not. Instead, it seems that for every sentence that you are asked to translate it has a corresponding list of acceptable sentences. (If you are a programmer, I'm speculating at a naive approach) Now because korean and english don't agree on things like definiteness or number for every noun there are at least two acceptable translations one with an indefinite article and one with a definite article. Then there are sentences for whether the noun is plural or not (because there isn't context). So if there were a one word noun there would be four acceptable translations but that doesn't even count any possible synonyms or dual meanings from one language to another. Then add adjectives and other nouns and you can quickly see how many possible alternative sentences you need just for one sentence.
Think of how many sentences you've translated up to this point. I'm sure you've run into points where Duo said the acceptable translation needed a 'the' instead of 'a'. I'm guessing that when the team of unpaid volunteers made this course they realized that to write all the possible combinations of acceptable sentences would leave the course unfinished for a long time and they would inevitably miss some. So rather than that, they could give every sentence one or two acceptable sentences then release the course in beta (where we are now). Then the users of the beta program could fill in the missing bits by hitting the 'flag' and 'my answer should have been accepted' options. That way it would only take a single moderator to verify that that sentence you typed is correct and could add it to the course at the click of a button. Given the thousands of people going through the course all those flags would build an optimal solution way faster than the six to eight people doing this in their spare time for free. As a beta tester, when we find an error like this, our job is to report it. When we make a comment to a sentence like this it doesnt raise any flags where the moderators can see it they don't know it exists. Instead save the comment board for cool cultural stuff, questions/comments about the grammar, or other things that could help other learners and add to our knowledge.