From the article:
The researchers caution that they haven’t established “a causal link between musical activity and the effectiveness of frontal brain functions.” They concede it’s possible that people who generally perform well on cognitive tasks might be more likely to take up an instrument.
Or a separate gene or experience or background makes one attracted or likely to attempt both playing music and different cognition-sharpening tasks.
Also, the sample size was 36, split into four groups. That's TINY!! 8-10 per group!
I checked out the original study, and saw a bunch of weird things. Most of the participants were female, many of them played different instruments, so who knows what difference gender or instrument made.
Also also, that test only measures an extremely specific type of thinking, which may not be linked to adeptness of other types of cognition.
The results seem completely plausible, but the study design seems pretty poor and the conclusions very shakily drawn.
This article critically examines previous studies that showed a link between working memory training (specifically via n-back training) and fluid intelligence, finding that the results may not have been as positive as reported owing to a number of factors including the use of a no-contact rather than active control group, and difficulty selecting tests that isolate the impact of working memory on fluid intelligence. The authors also present findings from a new study that show no improvement in fluid intelligence from dual n-back training, visual search training (active placebo) and no training (no contact placebo).
PubMed
Journal Challenged