I agree with your point about the value of appearing confident, and that it's difficult to fake.* I think it's worth bringing up, but I don't think it's a particularly large component of success. Depending on the field, but I still don't think there's really many fields where it's a notably large component of success (maybe sales?).

*I've encountered it. I'm an inexperienced web developer, and people sometimes tell me that I should be more confident. At first this has very slightly hurt me. Almost negligibly slight. Recently, I've been extremely fortunate to get to work with a developer who also reads LW and understands confidence. I actually talked to him about this today, and he mirrored my thoughts that with most people, appearing more confident might benefit me, but that with him it makes sense to be honest about my confident levels (like I have been).