I am surprised that someone might think it should. When I consider my extended family, more than half of them use two or even three languages on a daily basis

I am surprised that someone might think it should. When I consider my extended family, more than half of them use two or even three languages on a daily basis but in terms of “smarts” seem to be normally distributed, with no special correlation in this regard. So I’m unsurprised by the conclusion of a rigorous and larger study.