[JSoet]

I'm pretty sure it's referred to later in the article:

>All that distraction adds up to a loss of raw brain power. Workers at a British company who multitasked on electronic media – a decent proxy for frequent smartphone use – were found in a 2014 study to lose about the same quantity of IQ as people who had smoked cannabis or lost a night's sleep.

Still not a reference, but you could probably find the study given that information if you wanted to. Or I'm sure the author would respond if you tweeted and asked him or something. If you really want to find out I'm sure it's possible.