I don't know what your brother meant. But I can give you an example. When I was working I had a co-worker who was then my employee when I was promoted to be the manager of our department. She was really smart, efficient, tall, pretty, very well dressed, and QUIET. She had just been hired when I was promoted, but she was better qualified to run the department. It was really hard for me to get her raises or a title adjustment because no one would believe that she was so smart or hardworking. Mostly because she was quiet but I think also because she was pretty.
She eventually left the company and went someplace else were she had the same problems for the first year. People seemed to either be mean to her or ignore her. Then she got a new boss and she suddenly advanced, got employees of her own, and is now doing really well.
Assertive people do advance quicker than quiet ones, but if you're good at your job, things work out in the end.