Citadels’s market-making arm, Citadel Securities, IS extremely competitive in on-exchange trading. They don’t just do wholesale market making (I.e. service for a price), they provide liquidity on-exchange as well (much more competitive/latency/alpha based than service-at-a-price). They’re also very good at the hft taking.

Given, within Citsec, the retail market making arm is fairly segregated for compliance reasons, but it’s far from the only market making activities they engage in.