So what does this sentence mean? Is it, “(all) bats are not birds” or “those bats are not birds”? Do we use the definite article with plural nouns in Romanian to talk about all members of a group?
Yup, you should use the definite article (Liliecii) for both of those sentences in Romanian.
The Romanian sentence can thus be translated to both "(All) Bats are not birds." and "The (specific group of) bats are not birds." I think "The" is forced because the topic of this lesson is definite article for plural nouns.