Ok...though i think english and other languages have an absolut meaning when using determinative or undeterminative pronouns, whilst in german, i suppose, they are all depending from the word position in the sentence. "Das", I've learnt, could be even indicating a bunch of hippies, or anything else plural standing near me, as well as a singular being lying far...important thing is that it stands in first position inside the sentence. In any case Duolingo could sometimes have some bugs due to the mechanism of teaching a language from anew for the most of us.
I believe that "das ist" is a rather flexible phrase. Of course, it literally means "that is," but as you pointed out, it can mean "this is" or "it is." The frustrating thing about this is that Duolingo is just a computer program, so the nuance and flexibility involved with stuff like this isn't always completely captured.