I’ve seen claims that having settled funds doesn’t matter, that the brokerage must keep collateral at the clearinghouse and they cannot use customer’s money to do it: https://news.ycombinator.com/item?id=25981493. The clearinghouse suddenly demanded a lot more collateral for that stock (because its eventual value is so uncertain) and a lot of brokers couldn’t or didn’t want to meet it.

They also didn’t want to say anything that might give an impression of being insolvent (especially when it isn’t true).