There was a control group, according to the site of Stanway school. However the whole thing is very much a TV show, featuring a Big Brother presenter, the bassist from Busted, and a TV doctor who hopes it "kick-starts a national conversation about which aspects of technology use can HELP our children and which aspects are in fact HARMING them." No preconceived ideas there at all, right? But I can't find a paper, with details of the experiment, such as how participants were selected and how they were motivated.
https://stanway.essex.sch.uk/swiped-the-people/
The thought occurs that the participants were being goodie-goodies, pandering to adult concerns, and saying the right things such as "yes I feel much less anxious, also I want to pick litter, save some endangered snails, eat vegetables and be virtuous in every way because all the things adults say are so right, look how responsible I am, praise me".
I guess a better test might be an involuntary one, like a solar storm that knocks the phone network out.