User blog comment:Xilinoc/Ch. 662 (not released at time of writing)/@comment-26091912-20160211103131/@comment-1210805-20160211105745

It's not as though the Royal Guard have no contact with the Soul Society. They recruit new members from the Gotei 13, are tasked with slaying Menos, and travel between the two realms when needed. Ywach had also met Ichibe in the past, as indicated by their interactions, and likely placed him on the list based on past events.