From the description, it sounds like the routers have been reduced to more simple muxers, where a small tag added to the packet determines which port it goes over instead of trying to make a complicated decision all in one focal point.

This has pushed the decision back from the routers to the servers, where it scales with the number of hosts handling connections and can additionally be determined per /session/ instead of per packet. That state is held where sessions already need to be stored anyway instead of being duplicated in a router where it's entirely a burden.