I don't know for sure but I don't think there needs to be a chinese firewall between the two like at banks. Banks investment banking and advisory divsiions have access to insider information by the nature of their work, which is why the chinese firewall exists between them and the sales and trading desk. Citadel is a hedge fund, however that uses only public information (presumptively, as otherwise would be illegal) so i don't think there's a reason why they would need to be firewalled off. Also Citadel does use Citadel Securities for execution.

Additionally, they're both owned (mostly) by Ken, who is know for being a bit "detailed" (to put it nicely) when it comes to his businesses. He almost certainly has live second by second pnl updates for all parts of all of his business, he almost certainly was the one who made the decision to invest 1.4+ bn into Melvin, and he almost certainly knows that retail flow (esp robhinood) is a huge part of citadel securities business.

Honestly I think reddit has some grounds for a case here, but IANAL.