It sounds like the photographer in your linked article is advocating deliberate practice. He is advocating working on improving specific skills in a realistic s

It sounds like the photographer in your linked article is advocating deliberate practice. He is advocating working on improving specific skills in a realistic setting. Personally, I find side projects one of the least efficient ways to improve. I already spend 8+ hours a day "doing" at work. In contrast I spend less than 30 minutes a day at work learning and understanding theory, new concepts, and different approaches to a problem. So I get a lot more value from focusing my "free time" on the latter.