Even if their conclusion is correct, which after a quick skim I didn't see how they measured "cognitive" skills the result is kind of dumb.

Working on math problems might not make you a quicker thinker overall, but it will teach you math! The specific skill you enrich will become stronger at the expense of the rest of your life. I agree that time spent not socializing damages the ability to socialize (it's a joke among engineers in school). Clearly we are not making great tradeoffs, but I'm not sure this result helps us very much beyond what we already knew.