Though the better you get at organizing your code, the less you need to push your working memory. In the first decade of my coding life, it took me a while to 'warm up' and 'cool down', distractions were extremely irritating. In the second decade I started to find it easier to jump in and out of the coding state, by maintaining a more focused problem-space.
I am of a similar age to you I am guessing. I think many people have a mythical view of their own ability to escape the typical human experience.
Perhaps distractions don't damage your productivity, but I am guessing you're merely more productive now, so the short bursts you make are more productive than your old longer bursts, therefore your 'Warm up and cool down' periods FEEL shorter. It's not like people do nothing when not in flow, they just do less.
