"Ils sont pauvres, et ce depuis longtemps."
"They are poor, and this for a long time." Just doesn't sound right.
In this sentence, "ce" is referring back to the predicate statement that they are poor (sont pauvres) but there's no straightforward linguistic way to capture the same referential meaning in English. Translating "ce" to "it has been so" accurately references the state of "being poor" while remaining in the present tense.
See the word map of French to English below (the braces correspond sequentially):
[Ils] [sont] [pauvres], [et] [ce] [depuis] [longtemps].
[They] [are] [poor], [and] [it has been so] [for] [a long time].
An alternative translation is, "They are poor, and [that] for a long time" which is clumsy English, but technically correct.
I know exactly what you mean! The translations are full of such clumsy interpretations, things English speakers would never say otherwise. I thought that was the whole point of Duolingo interpretations, to not translate the way computers would. What document was this from? is this translation the 100% answer? A better translation would be: They are poor and have been so for a very long time.