It absolutely is, and it works against anyone not in the club. I was once told by a GM in my company whom I knew outside of work, “you will apply for this job and get it. Then you will apply for that job’s manager’s job and get it. After that I won’t be able to help you.” Once I got outside his sphere of influence, I no longer had anyone advocating for me and haven’t been able to get any new jobs, lateral or promotion, since. To be fair, I’m not a great employee and have risen to the level of my incompetence; but I’ve seen other people get promotions who were not as good at the job as me because they do more of the company social stuff.

And I know that those networks definitely favor men in my industry. So you’re right, even if there isn’t outright, noticeable discrimination against women; strong bias exists.