I suspect anyone willing to do that is probably already healthy, given the amount of effort they're willing to put in.
A fair number of people get healthy by doing that as part of the effort, so, no, not all the people willing to do that are healthy before actually doing it.
> given the amount of effort they're willing to put in.
Willingness to put in effort alone won't make you healthy, alone; effort on healthiness can be, and often is, misdirected.
Also, willingness to put in effort isn't intrinsic and immutable; it can be dependent on being convinced that the ffort is likely to payoff.
It ought to be easy for someone to realize they are having too many sodas, or playing too much video games if they think in terms of e.g. what diet/activity restrictions they might impose on hypothetical kids. Those kinds of realizations are way more actionable and way more likely to make an impact than some dubious diet "hack"
It may seem borderline corny to ask people to eat their brussel sprouts and limit their gaming/tv time, but alas many don't even do that.
People often impose poorly chosen parameters on their actual children, so I'm not at all convinced that merely considering what one would impose on abstract hypothetical children will magically get people to correct answers.
Besides, as I mentioned, armchair parenting is just one "trick" to help one brainstorm towards a variety/moderation mindset. Chinese cooking culture - particularly from older generations - is another way to build variety and moderation into a diet. Some people motivate themselves with martial arts (where health, pressure to maintain frequency and balance between aerobic/anaerobic exercise are somewhat secondary compared to the desire to improve technical skill)
> get people to correct answers.
I don't believe there are "correct" answers. Even mundane things like climbing stairs to the bedroom have been shown to improve health. As TFA expands on, it's a question of how one can incorporate more healthy habits into their life. This won't accomplished with repeatedly reaching for the "one weird trick"; there are millions of ways to become more active/healthy... if only people toned down on addictive sedentary activities like social media and the conveniences of pre-made food.
Modern fitness trackers are pretty good at automatically creating activity journals.
What I did find the most interesting though, was how hard it was to stop. After only 3 months of cutting, it took me over a year to restore a non-obsessive relationship with food. I am comfortable with my weight now, and it was a very useful exercise that I would recommend most try out. You get a valuable understanding of how much work a bar of chocolate or a beer actually is.