Because her coach said that was the most likely explanation, and her coach represents an informed distanced opinion with expertise in the area.
All she has is a guy refusing a job offer. After that she talks to some radical feminist who decided it was because of her gender, and she, for some strange reason, accepts that explanation as truth and shifts all the blame to the guy.
WTF?
b) The coach, according to the author, made that call after she recounted the story. Presumably in a lot of detail, and after numerous other coaching sessions in which her coach would have built up a detailed picture of how she worked and where her weaknesses lay.