But only on specific stocks they selected? They still had liquidity for other stocks?
If I were a clearer, and Robinhood were my client, and they had some collateral in deposit with me and a ridiculously large long position in GME in excess of the posted collateral, and GME were suddenly extremely volatile, then I would consider requiring more collateral and/or telling Robinhood that they may not purchase more GME. Because my #1 job as a clearer is to make sure that, at the end of the day, all the money I am owed is actually there.
If GME crashes, then the total portfolio of Robinhood and its clients will lose a lot of value. In theory, Robinhood and its clearer has no skin the the GME game, but, in practice, there are quite a few examples of financial companies taking massive hits when their clients go under.
So yes, the clearer could easily be willing to let Robinhood buy more stock that is seen as uncorrelated to GME but be extremely uncomfortable with the size of Robinhood’s position. This is similar to how Robinhood won’t let its clients hold positions when they have insufficient margin to cover the anticipated possible loss.
The clearing house needs enough money from you to be able to go buy those shares in the open market 2 days from now if you vanish overnight tomorrow. If the stock is going up or down 2% a day, it's a lot easier to predict how much they would need than if it's going up or down 200% a day.
With new accounts starting with Instant capability, they are all on margin until the transfer clears.