I agree with you - I got it as a translation from French to English so it was easy. I am convinced however, that if the question had been phrased the other way around, and I had given ‘sandwichs américains’ as the answer, it would have been given as wrong, with the correct translation ‘des .... etc’, because they have been pumping into us the rule that there virtually always has to be an article, du/de la/des for the indefinite. Ok, rant over!
The fries are in the sandwich? Huh. These cross-cultural cuisines can go a little off the rails. I don't know how familiar people outside of Canada are with poutine - it's a Québecois thing, fries and cheese curds with gravy. And here in my Italian neighbourhood of Vancouver, there is a shop selling poutine pizza - fries, cheese curds and gravy on a pizza crust. Never really felt tempted to try it.
Hey, When I was a poor student in the '60s "Sausage butties" with an English banger, butter and HP sauce were a major part of my diet. One loaf, a dab of butter and a pound of sausages can feed 4 young guys. French toast with Rogers Syrup for desert Very cheap way to eat. I survived.
In French-speaking Belgium, 'Filet américain' is also a mixure of raw ground beef with spices (much like steak tartare) and it is usually eaten with pickles, raw onions and ground pepper on a crispy bread roll. So I would normally probably translate this as 'sandwiches with filet americain'.
This is so irritating - but in a good way because I'm learning that there is not just one correct answer, despite what Duolingo seems to think! When I used to live in Provence, "des sandwichs americains" were open sandwiches, i.e. half a tranche of bread with a topping on it. But it seems like in different parts of the Francophonie, it's totally different. And I don't know what an American sandwich actually is. And in answer to some of the comments, I'm from Scotland and people here just LOVE chips (fries not crisps, although crisps will be used to) in a sandwich - actually they will put anything between two bits of bread and voila, you have lunch!