"Ellos no dicen mucho."
Translation:They don't say a lot.
I think this is really supposed to mean "They aren't saying a lot." It seems in Spanish they use the simple present tense a lot when we would be using the present progressive. But "they don't say a lot" and "they aren't saying a lot" obviously don't mean the same thing though, as the former is describing a personality trait and the latter is describing what's going on presently.
Duolingo shouldn't be accepting hablar instead of decir though in either case, as it would be teaching us the wrong lesson, even though it would be a reasonable enough statement if the former translation is accurate (a personality trait), but if the latter is the better translation (that they aren't saying much now), then I'm not sure you can easily substitute talking for saying, because in English, at least in American English, not saying much is a question of the substance of what is being said, not the amount of words being spoken.
Hopefully a native Spanish speaker can help here.
Why does DL change things all the time? Don't say "much" - 'a lot" where's the consistency?