They were likely running old Tibco/RV systems (network) distributed across the network (common for 1990s to early 2000 trading systems), and replaced the system

They were likely running old Tibco/RV systems (network) distributed across the network (common for 1990s to early 2000 trading systems), and replaced the system (and hardware) with multi-core boxes, and use shared memory for message passing. Reduces internal latency from milliseconds to sub-microsecond.