It's a scapegoat boogieman. A tool of techies in SF to manage their cognitive dissonance.
There are things everyone else in the city hates the startup/tech scene at large for. There are also people progressives hate for their not-progressive-enough views, and some of them have the gall to work in tech. When either of those (or any other number of criticisms) are stereotypically "bro" (or close enough), the people within startups/tech can blame it on the "tech bros."
It is meant as a slur. But bros are just a convenient and useful stereotype to blame. If it wasn't them, it'd be something else. "Tech bros" just happens to be OK to openly deride, because the identity has no social power within "tech" (whatever that means).
