That's a common misconception or intentional smoke to cover up shorting activity that is published online. The real shorters do not use mainstream brokers. They use small brokerages that allow the illicit shorting of Penniland stocks. Some examples are:
Most are not based in the U.S. to avoid SEC regulations. As the availability of shares to borrow against may be hard to find for certain stocks, shorters usually have to open multiple accounts at different brokerages.