The implication is that there is some kind of interaction between low birth weight and twin-ness, such that twins are not affected by it as harshly as singletons. So low birth weight may have detrimental effects, though perhaps more so in singletons than in twins. Many researchers have come to the conclusion that it is not biological but social factors that are responsible for language delays. Several studies have now found that young twins receive less directed speech from their caretaker and participate in fewer situations where their attention is jointly engaged with the caretaker. A recent summary of twins' language learning situation and how it differs from that of singletons is quoted below.
Twin language is unique in several ways.
Research on twin versus non-twin language is sparse, particularly regarding comparisons during the second year for children speaking English. Moreoever, detailed analysis of twin language reveals some domains in which twins are more advanced than singletons. Reznick's conclusion is that the twin situation may not be different from the general sibling situation. Note that the effect is slight but significant because it supposedly happens quite consistently, on average. I hope this review does not make the situation for twins' language sound doomed and hopeless.