"They read the newspaper."
Translation:Mereka membaca koran.
It could be. And, with "ini", it could also be translated with "The read THIS newspaper".
(this = ini)
I don't know if Duo accepts "the" for "ini", or only for "itu".
I think you are right that, very often, without itu/ini or -nya, so without the definite article, it means "a/an", the indefinite article. As if it's very important to mention that it's a specific determined item, they mention it. But it seems also a contextual thing, and not an absolute rule.
Without any articles, it could be also a "the" that is implied.
Especially here, where it can be understood as an habit: he reads the newspaper everyday -> I think it would be translated without the article in Indonesian.