I think the main problem with this article was that MMM overestimates how easy it might be for people to just decide that they have confidence. Now, if they are genuinely a person who already has a wide range of skills at a competence level and more than one skill at mastery level, including competence in social skills, then, yes, it will be easy for them to be confident, because they actually do have the skills to excel in a bunch of different jobs and opportunities, to find those jobs and opportunities, and to convince people that they are the right person to pay the money to.
If somebody doesn't have the real skills to provide the foundation to their confidence, then either their confidence is actually hubris, or their confidence will vanish as soon as it comes to the time to actually do anything. I suspect this is where a lot of cases of people who say "Oh, I'd really love to do X!" but never seem to actually do it come from. Basically, they do genuinely want to do the thing, but they're not really actively looking at the opportunities because they're secretly not confident they're actually capable of doing the thing. For these people, the solution is not simply to tell them to "work on your confidence", but rather to get them to identify the actual skills they'd need to achieve their dream. Then they need to study those skills in their downtime, until they do have the skills, and they will then find the confidence has appeared. In conclusion, it's true to say that some level of money can be exchanged for some level of confidence, but it has to be true confidence, not false bravado.
The reason I say that social skills should be included in the skillset in order to achieve true confidence is that pretty much every money-making arrangement in modern society, other than inheritance and chance lottery wins, needs you to have some level of social skills. If you want a traditional job with an employer, you need to pass the interview and convince them to hire you instead of the 1000 other applicants. And if you want to break out of that and be self-employed or become an entrepreneur, well, now you are in charge of finding your own clients. So you definitely need social skills, because otherwise you're not going to have any clients, because you either won't meet anybody that you can even try to persuade to buy your service or product, or when you do meet people, you will accidentally put them off by being a jerk, or sounding unsure about your own service or product, or failing to bring up that you have a product and just rambling about something, or one of the many other social failures that can be committed by the socially inept. If you have mastery of some skills that people actually want to buy, then, and you want to have the option to do occasional consulting for money after you've retired, then if you don't already have social skills, you need to start working on them TODAY.
If somebody has the social skills but doesn't really have any other skills, I think that's when you get people like con-men and other smooth-talkers who somehow seem to get money without really providing much, and it comes across as unethical.
If somebody hasn't really got decent skills in anything AND has poor social skills, as is common with a lot of people fresh out of school or uni if they weren't in the popular crowd, didn't pick their subjects wisely or apply themselves properly, and didn't have any family members encouraging them to learn something actually useful as a hobby, then they should not only immediately start learning social skills, but also identify something they enjoy and have the potential to be good at that people will actually pay for (it's no good to be a master of watching Netflix unless you're planning on being a film critic), and start learning that too. Otherwise they will end up competing with everyone and their mother for unskilled minimum wage work, and possibly even failing at getting that. Often they know this and will therefore have quite a few gaping holes in their confidence.
TL;DR: If you have poor confidence, there's no point just trying to brainwash yourself into having confidence. First, look at whether you actually have skills, and if not, work on improving those instead, especially social skills, and once you start improving, confidence should follow. If you have a lot of useful skills, including social skills, and still have poor confidence, that's when directly trying to improve confidence might become useful.