The paper is tapping on a long-standing debate about the effects of being bilingual. For a long time, it was thought that being bilingual was actively harmful, and indeed it's been shown in studies that babies with parents who speak different languages to them are slower to reach some linguistic milestones. Later research showed that this was just temporary, and not only was there no negative effect, but bilingual kids tended to do better on some tests of aptitude than monolinguals. And now this study is saying that no, maybe they're not better after all.
Wikipedia, as always, has more info: https://en.wikipedia.org/wiki/Cognitive_effects_of_bilingual...