The device matters a lot. A phone sits in your pocket and takes zero effort to use and get on the internet with. No other device is this convenient which is why people are constantly on them. By comparison, a laptop takes far more effort to use casually and still can't be used in the ways a phone can due to it's sheer size. If two people are on a bus or train, and one is using a laptop and the other a phone to consume media, the phone user can continue to use it while leaving, while waiting for another connecting transport, while walking down the street, etc. Meanwhile, the laptop user has to put their device away in order to do those same things. The person on a laptop is significantly less connected than a person with a phone by virtu of it's form factor. The phone user can spend time doing trivial, meaningless things while the laptop user must plan out what they're going to do while traveling because the length of time their device will be in front of them is limited; there's no instant gratification there like there is a phone. Add to all of this the constant connectivity to others via cellphone whether by text, IM, or actual phone calls and it's really a whole different beast over a computer in any form.
This is why the video specifically focuses on cellphones. Nobody is whipping out a laptop at dinner; they're staring at cellphone screens. Nobody is walking down the block composing an email on their laptop, they're sending texts on their phones while walking. Nobody is taking pics of food with their laptop's webcam and posting it up, they're doing that on their phones.
The phone is the problem.