In English Those refers to objects unless you add a noun so you would have to say 'Those people are very smart.' 'Those are very smart.' The meaning of smart would be assumed to be as in smart dressed. E.g Those trousers are very smart.
I think that Duolingo's similar words dictionary is not really good. When I wrote "Acelea sunt foarte inteligente." it said that "inteligente" is not the same as "deștepte". I am romanian,so I am sure that my answer is as good as the one accepted by them.