I agree in the sense that context mostly dictates which word to use here. Staying specifically with English and this sentence about ice cream, I'm not even sure what the scenario would be where you'd use that specific sentence. If you wanted to buy ice cream from the store you'd say "a tub of ice cream" or if you were at the beach "a cone" or "a scoop". I've never come across anywhere where you can buy ice cream by the litre. Litre means milk carton or bottle and in this case you'd say "a litre". "One litre" would be if you need it on a recipe of some kind, which is a slightly more realistic scenario though in this case I would have expected a unit of weight than a unit of volume.
If the sentence has been about milk then yes there is a clear difference between "a litre of milk" (implying a whole carton) and "one litre of milk" where you need to specifically measure one litre yourself. For ice cream, I don't know that either of those scenarios are realistic to me and this is why I think both solutions for this specific example should be acceptable.
Perhaps both should be accepted here, but I still feel like a and one are different enough, that I would understand if Duolingo is more picky here. The main example I can think of is if I were to ask you, "Do you have a litre of ice cream" vs "do you have one liter of ice cream". A implies that that you have at least one, but could have more, whereas one implies that you have one and only one.
It may be related to the fact that we have already been given the vocabulary for "one," yksi, and repeated opportunities to use it. By not using it here the course is emphasizing that the indefinite article "a" is implied in Finnish, where to specifically ask for a single liter --linguistically achieved in English by using "a", which is not an option in Finnish-- would in Finnish need the deliberate use of yksi.
Languages are rarely one-to-one equivalents. This is a good example of how Finnish and English are fundamentally different. It is also a good illustration of the complicated differences between interpretation and translation.
They mostly mean the same thing, but the emphasis is still different. If I say "one liter" that implies I want one and only one. "A liter" implies I want a liter right now, but maybe I'll want more, etc. It's like using a synonym, in a way. The same point gets across, but if you want an exact translation, adding the word one here is incorrect