I had a friend in college who was the ultimate expression of this. If he was in a line, waiting for someone, outside a professor's office hours, etc., he was working on SOMETHING, usually getting ahead of some reading for class. I asked him later, and he gave quite a compelling account of how if you truly added it all up, it had a pretty huge effect in how long it took him to get through his work. He was incredibly bright, went onto a PhD at MIT, and was also very sociable, which I suspect was helped by this strategy of aggressively seizing on these little breaks of time.

I need a good chunk of time to settle into "productive" work, even if it is just reading. I suspect that what is needed is a little bit more discipline at first and slowly it gets easier, but I just never had the ethic to stick to it, and because of this friend I don't even have the ability to claim any doubt as to how impactful it would be.

Genetics also plays a significant role here. For example, one of the major symptoms of ADHD is inability to quickly shift into productive mindset.

This is how I fight procrastination on certain tasks.

For some reason, a forced time constraint based on external pressure motivates me enough to finish a task.

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.