The idea is a very necessary factor, but yes not by itself sufficient. The Paul Graham article "How to Start a Startup" assumes that a startup is only developing something to improve on an existing idea/technology.
A startup inventing a product would be taking a huge leap of faith by believing that a power broker like Y Combinator would not be tempted to (in the event they came across an idea that they felt was revolutionary) to not farm out the engineering/business dev to seasoned developers; especially if they lacked faith in the approach/abilities of the founders. After all the process of execution/development is pretty much a commodity task.