I guess the premise of his comment is that there is a finite amount of money to be invested across all YC participants each season and that “back room” deals prevent startups that are less connected from having the same opportunity. Demo day could be seen as a way for all startups to compete for that finite investment on equal terms.
That’s not to say that I agree with the premise that it’s a zero-sum game. At the same time I can see eager investors losing interest in hearing from other startups after securing investments in the hot startup earlier on. A lot of startup investing isn’t merit-based, it’s more about hype and gossip than you’d expect.