It would be a lot simpler if the drop down vocabulary gave the correct meaning if they are intending to use something more uncommon. The meanings listed were "intend/are intending" and "shall", not a hint as to "let".
duolingo's dictionary is severly lacking and unless they license some external dictionary or hire an army of experts, I don't expect that to change any time soon. It's a good idea to double check with a real dictionary. Pons is quite good as it provides a lot of context information.
A dictionary will not care of this problem. We are dealing with conversational German. There is a need to explain a new verb, a tense and its context before dumping it in like it is right now.
So if i parse this as "yes they should open it" and it is accepted by duolingo as correct, then what is the german version of "yes they should open it" because the sense in both sentences is very different in english.
I agree with cephalium. If we're going to be penalized for wrong usage, there should be some some hint or guidance; ogherwise, we're not responsible for the hit. Nice to learn, but not in this uncaring way.