Full disclosure - I haven't actually watched the video, but I've read this, and this is the thing which I was referring to. It quite astutely details the marketing moves Nintendo made (which are plain to see) to swing the focus towards boys.
Yet that article also paints that it wasn't marketing that made the situation, it was already their and marketing just amplified it.
The industry did the math. Companies like Nintendo aggressively sought out people who played their games. It began publishing its own video game magazine, Nintendo Power, which had enormous outreach and allowed the company to communicate with its customers. Publishers traveled to cities, held tournaments and got to see firsthand who was playing their games. "That was probably the first age of game demographic enlightenment," says Mika. The numbers were in: More boys were playing video games than girls. Video games were about to be reinvented.
And the roots of sexism predate the NES:
Carol Shaw was the first female developer Atari hired. She is best known for designing and programming River Raid for the Atari 2600 at Activision. She says she never got the sense that the games she made were for one gender or another, and there was never a mandate from higher-ups to target a certain audience. When she interviewed for the job, she didn't believe she was at any disadvantage because she was a woman, nor did she feel that video games were the realm of men. She knew not many women held bachelor's and master's degrees in computer science and engineering,
Bold added for emphasis. sexism in culture at large had already labeled technology a "man's" job one that would have a knock on effect of keeping woman out of the field, adding to the idea that it's not for woman and setting up a situation were the people making games would be men, making the kind of games they found fun adding to the cycle.