How good is independent variable control in this study? The most common reason for bilingualism is things going very wrong in the birth country for either you or your parents. In which case, cognitive disadvantage is expected due to malnutrition, preventable childhood diseases, parents not being best adjusted to host country culture and education system and so on.

On the other hand, voluntary bilingualism is a sign of opposite things. Even if it doesn't have an intrinsic cognitive advantage, your parents were thoughtful enough to provide that for you, which comes with a slew of both cultural and heritable factors. Your schools being able to offer successful foreign language programs is a sign of relative affluence. Plus, say if your mother tongue is English and your learn Mandarin. You are now able to better communicate with a billion people and have access to their opportunities. It's hard to imagine how this is not a practical advantage in life, any artificial benchmarks notwithstanding.

I would allow that becoming bilingual in English, Spanish or Mandarin is preferable to learning something that gives you less additional reach compared to just speaking your mother tongue, where benefits are harder to quantify.