I don't really understand either. I understand why Duo has to phrase it that way (because singular vs. plural), but since that word is there, it shouldn't count as a mistake if you translate it. In my book.
However, the correct phrasing would be, "Seht ihr uns alle?" (in which "alle" does still refer to "ihr", but could just as well refer to "uns": "Can you see all of us?")