"Aren't you swimming with us?", I wrote. The software insisted that this was wrong, and offered, "Don't you swim with us?", which is how a foreigner might put the question, and thus instantly to be recognised as not a native English speaker.
I would go ahead and report it. Duolingo normally accepts the present continuous as a correct translation and should here as well. Same for the contracted form "Aren't you swimming with us?" which would be the most natural English phrasing.
That would normally be a good conversational equivalent here, but the software might not accept is as the word order is not interrogative. I don't think algorithms recognize tone-of-voice as a question marker.