Passing is a complicated subject for transwomen because attitude is critical. But at the same time, as Auroramarianna notes, actual looks, mannerisms, and voice have direct impact too.
I've met transwomen who were very nervous, very self conscious, yet otherwise visually should have passed and yet didn't. I have met women who are borderline in many respects (myself, face, til I complete FFS) but who get accepted because we just go about our business fully believing we should be where and who we are.
The reason confidence matters to transwomen and not cisgender women is because transwomen are often unlearning a mass of things pounded into them as "males" and rapidly learning new things that cisgender women have already culturally absorbed, often over the course of decades. Confidence becomes that bridge, but I do agree with Auroramarianna - if our looks, mannerisms, appearance, and voice aren't right to start, no amount of confidence will fix that.
Confidence is the icing on the cake. But first you do have to bake the cake.
