Mindfulness is not "focusing on nothing". Mindfulness is, if anything to "defocus". To let the mind flow and pay attention on what arises without getting caught up in it.
One of the favourite analogies I've seen uses is that of a series of boats racing down the river. Normally you're on one, and you might move from boat to boat. Mindfulness meditation is stepping off onto the shore and paying attention to the boats racing past.
"Focusing on nothing" is concentration practice. You need a degree of concentration practice to calm your mind enough for mindfulness meditation, but mindfulness meditation itself is not a concentration practice, but an insight pratice.
Of course, what they really need to do is put their phone away and actually focus on, you know, nothing.