It is clearer that they are different when you have more things to count, but with larger numbers it doesn't change. Let's say I ask you to go into a room full of men and count the women that are also in the other room. You go in and count: 'ekahi wahine, 'elua wāhine, 'ekolu wāhine. You come back and tell me: 'Ekolu wāhine. But if there was only one, you would count: 'ekahi wahine. Finding no more to count, you would come back to tell me: Ho'okahi wahine.
As I understand it, 'ekahi is for counting and ho'okahi is for stating how many. Presumably, you are not counting one woman here, but stating how many women are here. Though, since Duolingo lacks context, I suppose it is possible to interpret this as counting one woman rather than stating that it is one woman.