I bet that people who advocate for showing "vulnerability" are modeling this as a facet of strong confidence, and not opposed to it. But the thing is, if you really have reached the level of effortless confidence where that's a realistic prospect, you won't need that advice! You'll just be able to intuitively calibrate how much "vulnerability" to allow others, as a direct outcome of that strong emotional stability. Most people would probably be better off being told to be a little bit more guarded about their emotions.