I think YC is making a statement here: talk is cheap and ideas are cheap, but people are really expensive. If you want a solid company, the people matter more than the idea, because the idea will change over time anyway.
I know a lot of people who think that they have to have a "stroke of genius" where they think of something innovative before they can be a successful entrepreneur, when a lot of the most successful companies really did reinvent the wheel (they just did it better!).
The idea for a company is an artificial barrier to becoming an entrepreneur, because the idea isn't nearly as relevant as the execution (It's simple: what makes Apple or Google special, or more specifically, what made them special when they were starting out?). It almost always boils down to their approach to a problem instead of just the problem they were trying to solve.