WeBull CEO said that their situation was very different, as Robinhood is self clearing and they are not. They were told to stop trading. So the situation is not as simple.
While it's true that Robinhood has its own clearing firm, Robinhood securities is a NTCC member and is perusant to the same deposit requirements as everyone else.
Just as Apex clearing had to raise capital deposits, so did Robinhood securities. That they own the firm just means they wouldn't pay the clearing fee in return for funding the clearing firm.