There is a second version of this question which did accept my answer utilizing "Australienele . . . ." instead of "Australienii . . . ."
I would imagine that is because Australienii would mean the (all male) Australians or the (male & female) Australians. Whereas, Asutralienele would mean the (all female) Australians. Since both male and female austrialians speak english, strictly speaking this sentence should only use Australienii. But since Duo is trying to teach us a language, it wants us to know that when referring to 'The australians' either word can be used depending on context. Hence why it would accept both versions
It all depends on whether the sentence is intended to refer to Australians in general or a specific group of Australians.