GenZedong

That's an interesting take on it and one that seems like overall a valid way to make use of the phrase in practice, but I've never gotten the impression that most people using the phrase are talking about overcoming imposter syndrome. I'd think if they were, they would be explaining that's what it is about and I have no recollection of any association between the two, so if people have done so explicitly before, it's probably not something I personally heard.

That said, I have to disagree on some of the specifics:

Vulnerability is nice but not if it is coming from low self esteem or unfounded emotions. If you don’t tell yourself that you are capable of doing something you will never be capable of it. Pushing your boundaries is the only way to find the true extent of your capabilities.

This seems like a confused understanding of what vulnerability is. Vulnerability isn't supposed to be pretty and cool and presented just right. The whole point of vulnerability is being real about things and that could include low self esteem, it could include worries, insecurities, or also hopes, dreams, goals, etc. It's something that some of us more than others struggle with, at least in part due to fearing being taken advantage of or used in some way, because there is a real risk with vulnerability that what you share might be used against you. This is part of what I'm touching on when I say "is the culture punishing that kind of honesty, I wonder," i.e. is the actual cultural norms encouraging people to use others' vulnerable stuff against them in one way or another.

So to invalidate vulnerability if it is "coming from low self esteem or unfounded emotions" is invalidating a critical part of what vulnerability is. And, in fact, I'd argue that the way you're framing use of the phrase "fake it til you make it" is an individualist solution to what could be healthier in a community capacity, if people trusted each other more and were more reliant on one another. It doesn't actually address what is causing the insecurity in the first place and it builds no bonds with other people over shared struggles.

If you don’t tell yourself that you are capable of doing something you will never be capable of it.

This sounds straight out of a motivational seminar and simply doesn't make sense. The part that follows, about pushing your boundaries, makes more sense. What you tell yourself doesn't necessarily have anything to do with it. A kid learning to ride a bike doesn't need to get into mental gymnastics about telling themself they're a good bike rider in order to do it, they just need to get a feel for it until it starts building in their muscle memory. They may try to avoid doing it at all if they believe they are incapable, that much is true, limiting beliefs are a thing and can be a problem, but the belief doesn't prevent competence itself, it just is likely to get in the way of action. I've heard it's common, for example, for people in software to have imposter syndrome. This doesn't mean software is rife with incompetents because of their beliefs about themselves. They're still good programmers, many of them, they just don't see it.

And the flip side of this is that it's possible to overrate your own competence too. Sometimes this can get you far because of charisma and the like, when you aren't actually that qualified, which is counter to having reliable expertise in a field. But if things are based on a ladder of social status and needing to climb the ladder to be materially better off, some people are going to take full advantage of that.