I think having a list of usernames that match the input and have spoken recently when you type @ and then some letters would be a useful feature. Pretty minor, but just throwing that out there.
While I'm sure these are completely valid truths in some startups, I can't help but feel that the author doesn't really know what it's like to be successful in that sort of environment. You can paint things a hundred different ways depending on your perspective, but if this sort of "culture" works for so many people and they're happy with it, I don't really see an issue. OP was simply not a good fit for the startup scene's culture, and that isn't a BS statement meant to bar out anyone who doesn't fit the mold.