The design of the arrivals level of T5 is such that it is almost impossible to segregate passengers from any particular inbound flight from others. Once off the aircraft they very quickly intermingle and coralling even one one lot of say 100/150 without using off pier stands and bussing them separately to a "clean" or "dirty" channel would be a nightmare. T5 arrivals peak involves numerous near simultaneous and overlapping arrivals. Just stand ad look at the flows and cross flows next time you go there.