The "impulse issues" are reinforced by the phone. I've very often noticed when I've set aside my phone for a few hours (the rest of the evening until bed or whatever) that I have to literally set it aside at some distance from myself because otherwise it ends up in my hand without my even consciously deciding to check it. It's like a reflex. And then once it's safely at a distance, I repeatedly experience the urge to check it whenever I don't happen to be very focused on something else. The frequency of that urge diminishes over the course of a few hours and is accompanied by an increase in my ability to focus on something else for stretches of time. Sometimes I've even taken a weekend offline completely, and the effects are even more pronounced, and they last beyond the offline time. This is what the author means by "attention is a muscle". It's not that there's some baseline impulse issue and the device is merely an outlet. The device is causing the issue, or at least exacerbating it.