Anything a person can do is a matter of self-discipline. I could not have slept for 36 days because of working 2-shifts and still "have the discipline" to go for a run. Slim chances, but it happens.
But self-discipline is much easier to muster in "easy mode", when your other duties are taken care of by an army of servants.
Talk of self-discipline without talk of environmental factors making people less tired, less stressed, and less concerned with basic life matters to focus on that discipline is moot.
And talk of self-discipline should take into account aggregates, to discover what's the baseline for most people under certain conditions, not point at some outliers that did it against all odds and say "Here, anyone can do it" (as if difficulty doesn't matter). That's like pointing out Usain Bolt and saying "here, anyone can run sub-10 secs and if you don't you just lack the discipline".
Do you think e.g. a-list actors who trivially lose amazing amounts of weight for roles have "more discipline" that the average folks who statistically fail much worse in their diets?
Or that they have personal chefs, trainers, massagers, cleaners, and so on, plus a 5-10 million dollar incentive (to get the next "action" part), siding with their "self-discipline).