I doubt they were doing deep work in 3 minute chunks in line at the parking ticket office. One thing I realized for me is that simply priming the pump for later had non-zero benefits. Eg, doing a Google search for something, and just reading the result snippets counts for something in those 3 minutes. Reading the Wikipedia page on something isn't full actual proper research, but reading it five times (because you keep getting interrupted in the post office), but still managing to read it, counts as progress for later. Your brain simply just needs time to stew on things, hence the solution striking during a morning shower.
I think we don't give the subconscious enough credit for "getting things done" so to speak.
Since youth I've had (what was always termed a bad habit) the habit of jumping into a task and then never touching it for a week.
For sure there was constant worrying and ruminating on the thing I need to do, but I also have my mind ample amounts of time to 'sleep on it'. So when it came time to sit down and finish the thing, so much of the thinking and ideating had been done and I simply had to convert that into mechanical output.
And much of a project, like life, isn’t deep work. It’s the thousand little things, things which are indeed doable in the interstices