The thing is, those financial habits matter, but they don't make a millionaire on their own.
I have always been frugal and always saved. What I missed as a young person was knowledge of how to build and develop a career. Those things were actively discouraged for women in my family and community, and my college education did not include anything useful in that regard.
People will tell you over and over, "you can get a good job with any degree," but what they don't tell you is that you won't if you don't know how to make the right connections and understand what kind of work is actually out there and brings in a lot of income-- things that are completely non-obvious if you come from a lower-class household.
Overall, I was making progress and would have been financially fine and on track to retire at a normal retirement age, but I had a major financial setback at 40 that brought my savings to near-zero. Now I'm trying to rebuild with a mommy-track resume, a kid, and an ex with major mental health problems.
I have the same objection to the book Grit-- studying successful people is only part of the equation. There are lots of hard-working, self-sacrificing, frugal, gritty, sensible people out there who are never going to be millionaires, for all kinds of reasons, but mostly because they're in jobs that will never earn enough.
And yet studying successful people is the only part of the equation you can actually affect (discounting political activism). That's why it should be pushed, and people who respond as BDWW cited of "I don't take advice from people who aren't in the same hardship I am, only from people who are down here with me"....while their perspective may be understandable, they're only hurting themselves.
The point is that you can write down a handful of habits of millionaires, but if you give them to people who aren't millionaires as a prescription for how to become a millionaire, it may not work. You may be telling them things they're already doing. Because in fact, becoming a millionaire takes a lot of different pieces working together.
Have a Google and see how the replicability of the studies behind the book Grit turned out. It turns out lots of people are very gritty but never become successful. That's what I mean by studying success not being enough. It's similar to saying "thin people eat breakfast every morning" or "thin people take walks." They very well might, but so do lots of people who aren't thin, because neither of those things is enough on its own to produce the desired effect.
You can give people lots of helpful tips, but it's not the whole picture.
First, I was responding really to something you didn't say, and that's on me. I was conflating what you said, which was specifically for about becoming a millionaire with my take on a general "getting to a better financial state and closer to FI" point of view. Those aren't really the same thing and that's on me.
I will say, though, that I don't think we're really in that much disagreement. I agree that there are definitely situations where grit and determination won't get you through to full financial independence and certainly not to being a millionaire.
What I do have a problem with (not that you are necessarily supporting any of the following) is the corrosive mindset that comes with the arguments that were presented in the OP. They come with the mindset that it's all out of my control, and that mindset is there in people making part-time minimum wage to people who have household incomes well into the six figures. That mindset is that my choices don't really matter, there's nothing that I can do to improve things in a real way, so who cares?
It also seems that the most vociferous people who espouse this mindset and belief that there's nothing I can do and everyone who has succeeded has either been gifted it all or screwed others along the way to get it...are also the ones who are doing stupid things. Someone I know who always spouts out this stuff is also a person who voluntarily choose to move hundreds of miles away to work at a job that sounded interesting for low pay. He then complains on social media all the time about this kind of stuff. He could have stayed within a 30 mile radius of where he was at and be currently making, easily, mid 40's in a LCOL to VLCOL area. He shot himself in the foot as do thousands and thousands of other people each day.
The other thing is, using your analogy about thin people is good but it could be extended. Sure, you can eat the same things and exercise the same amount as others and not get the same results. However, if you eat healthier and exercise, you will improve your health - similar to how you will improve your finances if you live by mustachian principles. You just will. You won't get to the same heights as others, and some people will barely be able to move the needle depending on their situation, but there will be improvement. The one sure fire way to lose out is to complain instead of trying, and until you try, you won't know how much you can improve.
It all comes down to big picture/small picture. If you're concerned with big picture inequalities, feel free to be active on a societal, political level. Changes might eventually be made.
However, very little positive can come out the mindset of giving up, things are outside my control, everyone who is better off than me cheated to get there and therefore I can never get there on my own.
ETA: After thinking a bit more, it comes down to this for me. No one needs to hear that a single person with no assets, outside support, or other sources of income making $20k a year can retire in 10, 20, or even 30 years with a million dollars. It's not going to happen, and everyone realizes that. What many people don't realize is that a single person making $40-50k can make solid financial decisions *in many situations* and actually be on the path towards becoming financially independent. People don't see that because they've never experienced it, know anyone who has, etc. Therefore, that's the perspective I'm going to choose to focus on. Things inside our area of control and the fact that positive change is possible for many people.