Robinhood doesn't do scams, they're planning for an IPO next year. They aren't running off with your money. They just counted their chickens before they hatched. They thought that it would be covered under SIPC because it's basically an extension of the rest of Robinhood is. The SIPC is saying that they only cover accounts holding money that is reserved to buy securities, not just a general savings and checking account.
Robinhoods idea is that it would serve as both, hence why you have to have a Robinhood account to get an invite but the SIPC concerns are having to insure the people that will just use it as a checking and savings for things other than securities. Good thing about the waiting list is that they'll have time to come to an understanding but it'll be hard now to get the FDIC on board because they've shown their hand and the banks will do everything they can to fight this.