The problem is that it is English that is confusing not Duo.
In English conversation, it is not necessary to distinguish between some and all although it might be confusing if you don't. In French conversation, you have to even if it is clear which one you mean.
In English the burden is on the listener to determine if the speaker means some or all.
In French, the burden is on the speaker to make it clear which one he means.
I recommend that you get in the practice of translating the French as it is. Always include the determiner for your own benefit if not for your audience. If you are aren't crystal clear about the English, you will always have difficulty with the French.
Because English speakers routinely fail to make the distinction between some or all, many of them aren't even sure exactly what the difference is in speech. If you aren't sure whether you mean some or all when you say I like music, then you won't translate it correctly into French because the French statement requires that you specify which meaning is intended.
But this is how Duo works. You come across a translation that doesn't seem to work and you go the comments to find out what is going on.
There is nothing wrong with Duo providing examples where typical French conversation does not necessarily translate well into typical English conversation. The student finds he can drop some when translating in one direction but gets an error when going in the reverse. The student goes to the comments and finds that while some is optional in English it is always required when it is appropriate in French.
You can go to about.com or some such and find a detailed explanation but a couple of months later it will be buried in your mind under a ship load of rules. Then you come to Duo and get your fingers burned on the keyboard for forgetting or not knowing. Then you remember.
Think of it as the nun in those movies about early Catholic schools where she rapped the students knuckles and yelled wrong, wrong, wrong in front of the class.
Of course, in this case only Duo knows you screwed up big time and all that happens is the green owl sheds a tear. And you have to do the question again, often algorithmially frequently.
After all Duo did earlier present the difference between des and les even if they omitted telling you how English is pretty vague about how it is used but French is not which presents a trap for native English speakers.
Here, they make that point with a simple two word example. You can say rich men in English but French requires at least one more word in normal conversation.
The next time Duo provides you with a similar translation, a bell will ring in your head. You will think ......ok, the frog element in Duo French is trying to strike at les Anglos (note that I wrote les Anglos because I have to specify all or some) by providing a semantic subterfuge in what seems like a very simple two word translation that is difficult for native English speakers like me. Apparently they hope that somewhere there is a nun waiting to spring out of hiding and treat me like they think I deserve. Well, they fooled me once but not this time. ......
Not so simple really.
Les also means ...all, the idea of something, every example of something in the world or class of something of something in the world.
Context tells you whether les refers to those particular people as you suggest or all members of a class.
The most likely take on Les gens dans la rue would be all the people in that particular road.
Alternatively, les could refer to those particular people that we already know about, that have already been identified in previous discussion one way or another.
In English, we say people are running in the street and leave ti up to the listener to decide if we mean....
all/everyone in the street is running
some people in the street are running
a group of people that we already know about are running in the street.
In French, you have to specify. You have to use the two options, les or des, to indicate which of the three is your intended meaning. Of course, you can restructure your sentence so as to avoid any confusion. But unlike English, it is the speaker's job to make it clear.