I also think these are platitudes. In reality, no one is allowed to be "just themselves". But changing yourself to be more likeable (especially in social situations) is such a conflicted topic.

At least in western cultures agreeableness has a bad reputation and is not encouraged in boys, imho. As a girl you are raised to be agreeable and it is called "nice". I believe men and women have a different understanding what that means because of how we are raised differently.

In my experience some men seem to think nice means basically to avoid conflict and be especially generous (not necessary in a materialistic way, also offering help etc.).

While for women it means to be sensible to your partners feelings and plan accordingly.

These are two notoriously incompatible modi operandi: a "yes man" who hides or doesn't reflect on his feelings and wishes. And an increasingly controlling woman, constantly guessing and overinterpreting what their partner could possibly "really" be thinking.

A lot of women would rather choose a partner who is less work. Even when that means he isn't as generous. Therefore "just be yourself / confident" has a grain of truth in it. Just not in the way people might think. Another truth I had to grow quite old for to understand.