I don't think this is the same as what you're referring to. Under the DTC rules, this would likely have been an "Additional Participants Fund Deposit". The rules seem intentionally vague about what happens if you don't make this payment. As far as I can tell, the rules allow the DTC to unilaterally demand an uncapped deposit from a participant at any time.

Fidelity has $3.3 trillion AUM. Imagine if all its customers decided to take some actions that led the DTC to demand an "Additional Participants Fund Deposit" equivalent to 100% of what's at risk, like in Robinhood's case. Would Fidelity's inability to pay that be viewed as due to a failing on Fidelity's part?