Maybe a a very particular context, like a literary one. Probably not an everyday one. I don't see a rationale for using this sentence. Duolingo courses often have quirky sentences. While " this is one " is acceptable here, the translation up top says: She is one.
It sounds to me like the sentence is referring to placing the object / female in some sort of group. For example, if you had a group of people, and somebody asked you to identify the vegetarians (or whatever), you would say 'she is one, he is not one, she is not one, he is one' etc. :)