I'm pretty sure a few of those wake early simply because it is expected, not because of discipline. They aren't the only ones that have discipline either.
1. verb "train (someone) to obey rules or a code of behavior, using punishment to correct disobedience."
2. noun "the practice of training people to obey rules or a code of behavior, using punishment to correct disobedience."
So yes, having to wake up to make it to a job at a certain time or else you get in trouble is actually the very definition of discipline.
If people on my team are constantly getting to work late and I have to talk to them to come in early, I've disciplined them and they now change their behavior because of it (come in earlier).
You don't gain discipline by waking up earlier than necessary. If you don't need to be at work until noon, waking up at 5am doesn't add discipline to your life. Waking in time to go to work is discipline, but if you don't need to be at work until 9am, there is no need to wake any earlier than it takes you to get to work on time. Waking early for the sake of waking early or having general "discipline" isn't such a thing.
> You don't gain discipline by waking up earlier than necessary.
You absolutely do. Please take a second to actually lookup the definition of the word. It may not be added discipline in your life, but it is for many other people that don't live with the same daily habits as you. You don't get to change the definition of a word to fit your narrative, sorry.
The point is that waking early is not inherently a thing of discipline. You can no more judge one's discipline by one's waking time any more than you can judge one's discipline by whether or not they play an instrument every day. Both can be part of a disciplined life, but neither are actual markers of it. Neither are going to suddenly give one more discipline in their life just by doing that one thing. It isn't a magic cure. This is definitely not changing the definition one bit - you have already provided it.
Yes it is. Ask any engineering manager.