I accept that some people are doubtless much more talented in acquiring languages than I am. But I did take 4 years of high school French and was considered "good." And, traveling with a friend in Paris, I clearly knew a lot more of the language than they did. But I'm a very very far way from being fluent.
Maybe the people who claim to spend a few weeks with Duolingo and get the language are uniquely talented. Or maybe they're BSing.