Also nobody really has even a 1% chance at becoming a 100B company. There have been very few of those in human history, the odds are << 1% even for a top YC company.
You're forgetting that they are talking about the companies getting $1B valuations, not all startups. If you line up 100 companies that are Instagrams, Pinterests, Facebooks (when it was valued at $1B), etc, one of them getting to $100B can absolutely be a 1% chance.