You can open an Ameritrade or any other brokerage account and deposit money into the account with no intention of purchasing securities. There is no dispute that the SIPC would cover your account in that instance.

The only thing that has changed here is Robinhood is explicitly marketing their brokerage account as being able to be used as a savings account without any need to invest in securities.

It’s not black and white on either side. If there ever was a default event, it would surely go to court and it’s not 100% clear who would win. For that reason, I wouldn’t make use of the account.