Is the sandals with socks thing some sort of deliberate middle-finger? In NYC on a stage, no less.
I think it has more to do with maintaining a kind of legitimacy/authenticity within the hacker community than anything else. It's just good business sense to be the opposite of what geeks don't like/understand: VCs in suits. When you're in the business of funding businesses, you want to be approachable.
Birkenstocks are a kind of footwear. Lots of people like them. What's the point of getting affronted by this? Does it make the world anything other than worse?
Understanding that people judge you by how you dress isn't snobbery, it's accepting reality.