As someone who finds it impossible to change their sleep cycle and has tried almost everything in that regard & read a very large chunk of the scientific literature on sleep, both your view and the view of the article seem naive.
Maybe the GP is a natural early riser. I'm not but have managed to orient earlier. If left to my own devices 4am or 5am bed times are the norm but as I work I am up before 8am feeling fully rested and on an 8:15am call every morning. I must get 7-8 hours every night. That is the main differentiator.
Do you have kids? I got a baby this year and my sleep pattern completely changed: I used to sleep late into the morning but for some reason I now wake up by myself at around 7.30 every day. It's a radical change and it's apparently something a lot of fathers experience.