This is a self-reinforcing concept as well; I haven't read the other book mentioned (The Anxious Generation, I put it on my wishlist) but I'm convinced that it makes mention of this as well, that is, not being on your phone in these interstitial moments is in itself a cause of anxiety, not so much because of the places your mind wanders but because of its constant presence and repeated mini dopamine hits. Addiction, basically.

I've got a vacation planned, I should make a point of it to leave my phone alone for distraction / entertainment / interstitial moments.