Interruptions are a good theory, but it seems unlikely they would have people use their own phones for the experiment, as that would introduce as many variables as there are participants. I would think they just hand everyone a 'clean' phone with no apps on it.

I'd love to see the difference between someone reading a phone-sized piece of paper, a phone, a non-touch LCD that only shows a block of text, and a similarly sized e-ink screen.