Can you sell a stock you don’t own in CDP account?

Recently, read a book called “Rich by retirement” by Joshua Giersch.

He mentioned that brokers can’t see what’s in your CDP account. Since the broker dosent know what you own, it doesn’t have any way of knowing what you can safely sell, so it’s entirely possible that you could end up selling stocks you don’t own. Anyone can explain how this works? Also, what if you buy shares using CDP but don’t pay?

I use Custodian account so I’m not very sure about CDP.

Thank you!