A lot of engineers have a good bit to say against the existence of widespread sexism in engineering, myself included. Engineering in computer science has long been represented by a nearly nonexistent barrier to entry outside of one's capabilities and their relevance to the position. Even traditional, and technically very relevant, lateral predictors to output are such as formal education, are largely ignored. Your accomplishments and capabilities interviewing are ultimately what get the hire, with very few exceptions. Anybody who has been in a hiring position can speak to the utilitarian pursuit of the placement; race and sex are the last thing on the mind come hiring time.
All of that being said, however, I don't find it remotely hard to believe that Ellen Pao's recounting of her experience in the world of venture capital is far from the truth. I'm actually relatively certain that she's spared us a good bit of the details. But this isn't engineering, this is finance; quite rarely about the utility of any particular individual in a role, and almost entirely centered around pretty horrible characteristics. Cronyism is the most important characteristic in the club, trading favors, trading connections, looking the other way, getting away with this, getting away with that. The whole thing is a zero sum game, because nobody within is creating any value, you're only ever vying for a piece of the pie baked by the outsiders who actually produce things. As Pao points out, any partner you have largely considers you as a mechanism by which they are to have less investment capital available themselves, and any senior sees you as a way to bubble up the greatest picks for them to skim off the top.
The point is that the business is not about merit, it's about being in the club and playing ball. To deviate from the standards of the club just means you're less of a sure thing when it comes to being a crony, and it doesn't take much to understand why a woman is an outsider in a club like this.
So when we talk about sexism in Silicon Valley, let's please not conflate these two very different businesses. One of them is made up of worker bees, and we don't care what kind of bee you are as long as you're outputting honey. The other one is literally Wall Street pretending like it's anything but.