She should be taken seriously because many people erroneously believe that can't be true.
I've worked in tech for decades, and this doubt you express about a fairly typical-behaving young woman is not unfamiliar to me; and it's rarely proven valid, among my peers. Rather, it's almost always an expression of the jealous misogyny of the complainant.
_Every_ attractive woman I've known in this industry has had to suffer endless doubt about their abilities, beyond what I consider normal.