-the competence or skill expected of a professional.
-the practising of an activity, especially a sport, by professional rather than amateur players.
What the author is writing about is conformity. That said, calling someone unprofessional is often used as a way to make them conform, so her confusion is understandable.
And, since the "professional" workplace has historically been white-male dominated, that is the standard for conformity. Being not male and not white makes conformity all that more challenging.
Fortunately, conformity of presentation in the workplace is waning. That’s why corporate CEOs can wear t-shirts.
So, if you are a black woman, please wear your hair as you like. Be the change you
want to see. Take the risks and push back.
The only way to stop conformity is not go along with it. And, if you are a professional, you can go where people are accepting. Those who don’t want you because you don’t conform will be less competitive without you.