My understanding is that gender is how you think of yourself. If you consider yourself to be a man, then regardless of what society says, you’re a man. They can’t simply assign you the gender of woman. The terms AMAB and AFAB are referring to your sex, not your gender.
Trans is not just about your body, but also about how everyone and the society percieve you (that’s why you have multiple type of coming out).
So if someone was born male, and thinks of themselves as female, but they haven’t told anyone, would you say that they’re cisgender because the gender society perceives them as matches their birth sex?
And I would say that someone who has always been female and happens to have a male body as having a “locked” gender. I just use “cis” to mean that your gender matches your sex. Not anything about whether or not your gender is fixed.