Do you have evidence that Citadel is RH’s prime brokerage? Being an executing broker or a trading counterparty or paying for order flow has nothing to do with being a prime brokerage or clearing RH’s trades.
Before that they used Apex.
This commenter seems disastrously uninformed about something they have very strident beliefs about.
Nonetheless, all the conflicts-of-interest between Citadel, Robinhood, and Melvin still stand. Citadel own Melvin, Citadel are market-makers for Robinhood, Citadel are members of the DTCC, Robinhood users cost Melvin billions, Citadel are footing the bill.
You don’t need the connection to Citadel. If any member of DTCC blows up DTCC holds the bag. But all that means is all members of DTCC hold the bag, including the members holding the non-settled long positions because all public equities in the US trade through DTCC or one of its members.