A techbro is a bro who is into tech. That is the simple definition, but the development of both the term and the phenomena is vague. According to UrbanDictonary, the term was already recorded in 2013 (by an extreme coincidence, three days after my writeup for "bro" on here), but it had already been developing for years, and the development of the "bro" was necessary for the development of the techbro.
First, a little bit of history: there was a time when "geek" and "jock" were very different categories. Early computer and internet users, before 2000, were all in the first category. And as geeks, they naturally looked at the world from the point of view of the underdog. And also as geeks, and early adopters of the internet, they were curious, open-minded and usually a bit more culturally aware of the wider world. They had a variety of political and social beliefs, but in general were "anti-establishment" and irreverent. At least this is the story: note that from the beginning, aerospace and computer technology often went hand in hand with the national security establishment and big business, so all that Star Trek utopianism was nestled up next to working at Lockheed Martin.
Also, personally, my biggest immersion in tech culture came at Free Geek, a community and worker ran cooperative with an environmental mission. So my visions of that early internet might be a bit askew!
But one way or another, this view had a lot of validity: tech culture was a non-hierarchical, open-minded, inclusive mess that was about imagining a different world.
And while this was going on, the "bro" developed. As mentioned in that write-up, "bro's" often took countercultural activities like extreme sports or drug use and turned them into mainstream activities. And it was this development of "bro" that let the "techbro" exist. A techbro could take countercultural activities and turn them into something banal. A DMT-infused trip through virtual reality could safely be segued into a powerpoint presentation on market synergies the next day. The wildest disruptions of consensual reality co-existed along the crassest forms of corporate life.
One thing about the techbro is that while there are obvious examples of them being a real thing (both personally and in the media), tech culture and the tech industry still tend to be socially liberal, and international in flavor. In partisan politics, Silicon Valley is still a very Democratic place. The techbro, in short, might be notable because they are rare and incongruous. And of course, "techbro" runs along a spectrum, from men with offensive behavior to men who just enjoy a couple of stereotypical hobbies. And as is often the case recently, the "techbro" might be an example of a story superseding reality. Once the story has been created, male tech workers start identifying with the stereotype---partially out of defensiveness, and before long, start to identify with the stereotype. The story creates a reality.