Small startups often don't have access to a diverse talent pool to hire from since it is hard to convince anyone other than people you already to join the company. I don't think they deliberately refuse to hire women engineers judging by the diversity page[0] on their website
Until one person left recently (nothing acrimonious), the engineering team I'm on was 9 men, 8 women. At a startup.
The "pipeline problem" doesn't exist. The "we're too small to do that" problem doesn't exist.
You know what does help? Recruiting at places other than the Stanford CS job fair. Partnering with mentorship organizations. Sponsoring or hosting events that show off your company to people who aren't Stanford CS students. I could go on and on.
0: https://triplebyte.com/diversity