German from English
The formal verbal form with "Sie" should be accepted as an alternative to the informal "du".
... I'm not quite sure how to respond: Was this a comment about a specific Duo exercise? In general I agree, provided conjugations are correct and other related criteria are met.
If this was a comment about a specific exercise, then down at the bottom of most exercises is a "Discuss" button you can click on so that your comment gets linked to that item specifically. I really like that button, because it lets me see what other people think about the specific question I'm having issues with.