I think you are correct: 那个人是谁？could mean either "Who is that person?" or "Who are those people?" However, you could use 人们 to specify "people" (plural): 那个人们是谁？

Except "person" would be more normal in regular English. But report it if you think so.

That doesn't help

个 is a measure word used for people (as well as certain other nouns). This means that if you want to specify a number or use this or that to talk about a person, you must put the measure word inbetween them.

You should not say 这人，you should say 这个人. You should not say 三姐姐，you should say 三个姐姐。

You are assuming that the person is a man. 人 can be a man or a woman. 男人is more specific for man 女人 is woman

However, if you want that answer to be accepted, I suggest that you report it to Duolingo, or the contributors, and suggest your idea.

What about if it was a woman? Woman are people too, aren't they?

Apparently, one must already read and speak Chinese fluently in order to "learn" Chinese on Duolingo.

yes, because the key word here is 那 (na), which means THAT, not THIS, which is 这 (zhe)