"It is not necessarily true."
Translation:Non è detto che è così.
Sorry - you'd already said it was wrong. It's a bad idea to start a whole new concept, tenses of verbs, with an idiomatic saying that has nothing to do with the matter at hand and has an error. I can understand
Si dice che and the subjunctive is used because it is "subjective" so you've taught me something here! Duolingo just screwed up on this one. Actually you can usually tell something's wrong by the number of comments.
I had to pick out of words (vs typing) and with the words available to me I had to put together "Non è necessariamente così" . It was accepted. This is just a bad question I think. It doesn't work in multiple question formats, then they had to be overly lenient on what counts as a correct answer.