You're right, I misread it. I'm reading my own experiences into it, but those aren't everyone's experiences. I had what I felt was crippling social anxiety in my teens and 20s (I'm in my 40s now), or at least an extremely limited and lonely high comfort zone surrounded by an unlimited discomfort zone. I felt like my whole life had been filled with people always telling me to "just be more confident" or "be more outgoing", but what does that even mean? That's not actionable advice. It was especially annoying because it's so ambiguous and unactionable, but always seemed obvious to whoever was giving me the advice.
One thing that helped me personally was realizing that in my case, "just be more confident" really meant not worrying about putting up a falsely confident front. Like, I always felt this pressure to appear confident and charming and however I wanted people to see me, but I never felt that way on the inside, especially around women I was attracted to or other people I wanted to impress. So if I didn't feel confident, the only thing I could do was at least attempt to appear confident. After all, that's what everyone kept telling me to do, or so I thought. Anyway, this pressure (probably rooted in a form of narcissism) that I subconsciously put on myself in the moment to be confident/charming/etc. was just overwhelming, to the point where it sometimes rendered me incapable of social interaction at all. It was like the part of my brain that was in the driver's seat during social interactions would just lock up sometimes and I wouldn't even know what to say. But even when it was working it was exhausting.
In hindsight, this was the source of much of my social anxiety. It was like an ass-backwards subconscious instinct to play the role of the person I wanted other people to see me as. That's never going to work, it's like scripting and acting in a role in real time, a role that you don't understand well enough to write in the first place. I could never begin to get any better socially until I really truly internalized the fact that I didn't need to pretend to be someone I wasn't when I presented myself to other people, and I should just try to be in the moment rather than anxiously observing my own behavior. But, that's probably pretty specific to me and may not even make sense to anyone else.
Also my brain parsed "confident and assertive" into "extremely confident". When I think of someone who's consciously assertive, I think of a display of confidence for its own sake. Maybe that's not what they meant though.