The more precise cynical interpretation would be "work for a promising early stage YC company". Note that he also could have told you to work for a late stage one or apply to YC in order to start one. But it's probably true that working at a promising early-stage YC company is what would most benefit YC on the margin. (Although if what benefits YC most on the margin is what generates the most value, then generating more value for YC also seems like a good way to generate enough value that you capture a significant chunk.)